Room temperature isn't easy to change. I don't have (and never would want) aircondition, i.e. 28°C in summer. Below 23°C I get sick very soon. In winter the PC stands at the right side of the radiator (next to the window), which isn't a that cold place either. (To the right of it runs an audiophile 65W tube amplifier 14h/day.) I think an adjustable slow fan would make sense. It is ok when a fan is somewhat audible during 3D gaming, but it should stay silent when I record sounds with a microphone or use it as a VCR in the background. With silent but hot running components I always worry that they fail after 2 years. I think I will definitely install 2 power supplies (the old and very modded Fortron AT one cools the DOS part with a big hanging fan added by me) to run the modern PC independently. The mini-ITX(?) mainboard may be placed either upside-down at the bigtower ceiling, or on top of the AT power supply (there is some space above, see photo).
My current config (i7 720QM + dfi acp cp 330nrm + nvidia GT240) can run in winter without fans running but with good coolers (as i hate noise and getting low power components is the best way). And an i7 720QM isn't that slow.
A good start point is silent pc review to find what you might want.
When trying to make efficient/low power/low noise config, you'll need some basics:
- good air flow in the room (at more or less 20°C most of the time) if you want passive cooling.
- at least one rheobus to control the fans speed (as for security you'll need to put at least on or two).
- every component should be able work without fan => that imply buying industrial component (like my dfi-acp mainboard) because they can sustain higher temp. Mobile and most modern desktop cpu will also work and resist longer without fan.
- power supply should be very efficient (platinium label would be best). Totally passive cooling PSU exist but might not be the right choice as there are hybrid cooling one (a heatsink for passive cooling and a fan when the heatsink isn't enough).
- forget 3.5" hard drive as a 2.5" will produce a lot less heat. SSD might be an option if you want to build a very fast computer.
- pick good heatsinks (my favorite are thermalright's).
- get low noise fan (my favorite are noctua's).
- How much capacity have 2.5'' harddrives?
(1TB is absolute minimum, 3TB is better.) I especially want a quiet and *reliable* harddisk, i.e. means a boiling hot running tiny one that dies after warranty end is an absolute no-go. I don't trust in SSD not least due to limited lifespan. They only make sense for shockproof mobile devices or as boot accelerator with the same size like the PCs ram. I don't want to pay a moon price for that stuff nor buy cutting-edge ultra-highend hardware that anyway tends to cost half after only 1 year. But if possible, the graphics should have roughly PlayStation 3 quality (but I need no higher gaming resolution than VGA or maximum 1024*1024). Important is that the GPU should support also Linux.
First off lets look at what it looks like you have in that case now. It looks like you have 4 5 1/4 bays starting at the top with hard drive swap bay, then maybe a cdrom / dvdrom and cdwriter / dvdwriter, hard drive bay with internal 3 1/2 hard drive, 3 1/2 floppy kina floating in air(what happened to the bay?) and then the bottom of the case is 2/3 filled with the old board and above that is the replaced power supply with one big honkin led fan(So I'm guessing ATX20/24 jumpered on and then the motherboard cable either replaced with one for that old AT board or your using a conversion cable.)
First off lets talk of a few things. Do you leave that case open all the time? I would not recommend it as this messes up the normal air circulation in a case, it would seem open is cooler, but that's not the way it works, as with the case open the fans don't draw off the heat and expel it from the case. What normally happens in a case is air comes in at the bottom threw the front of the case and is pulled up over the board and drives and then pulled by the power supply fan and expel it out of the case.
It looks to me there is enough room for an ITX board to be mounted at the front part of the case next to the old motherboard, removing the plastic card guide(if this is used to hold the front case fan you can mount the fan to the metal of the inner case or mount the fans between the plastic front and the metal inner panel, if a regular fan won't fit, there are thinner ones, or you can use several smaller fans(replacement vid card fans come to mind.).I'd put as many fans in as I could if I was you. in at the bottom and out at the top. Plus put in a bay fan(get the good one with three fans.) in front of that internal hard drive in place of the filler panel.
Next is mounting a hard drive for the ITX board, if you have a bracket for that 3 1/2 floppy, more than likely it has an internal drive location or you can use a couple of pieces of tin between the floppy and a 3 1/2 hard drive.
Connections from the ITX boards ports would be cables ran to filler brackets, or mounted in the I/o cutout on that case, also you could cut slots in the plastic front panel for usb. etc. or use a 3 1/2 i/o bay if there is a second external 3 1/2 bay open.
Then to power the ITX board I would mount a power supply for it above the current power supply and cut a hole threw the case, you mite be able to use a single 12 volt power supply(easier to mount.) and an ITX DC-DC plug on regulator that just plugs in to the ITX boards power connector, these are used to run ITX in a car, etc. Your going to have to make a cable for the power use like 12 or 14 gauge(This is so the resistance is low and you will have little voltage loss over the length needed to go down to the ITX board.) stranded copper wire. You can buy short lengths cheep at most hardware stores.
Now for a cdrom, dvd, writer, etc for the ITX board I would just take one of the ones you have off the old motherboard.
I wouldn't worry about a floppy for the ITX as you can move files etc. by usb and most modern boards support usb boot, just slide the usb drive to write protect if using it to fix a virus, etc. Plus floppy's are just too small for modern software.
You can use a cheap KVM switch box to switch your mouse, keyboard and display around(as someone else suggested.) Velcro works good to mount one of the small KVM switches to the top of your case.. Use an adapter on the ITX if it only has display port and it will work fine with your VGA monitor or you could get a LCD display and use it with an adapter on the old board. Watch the really cheap KVM boxes as they sometimes have really short cables. I got one really cheap but cables were only like 2.5' long and barely fit with the switch sitting on my mid towers, I doubt the cables would work for that big tower. Remember if you get a four computer KVM(Not much more than a two switch box as they sell a lot more 4's than twos.) you could use one of the extra cables from the ITX board to the back of the case.
Those are some ideas of how you could rework that computer to a dual computer box, as you should be able to tell I've done a few franken computer upgrades over the years. Anyway have fun.