Good points by both. Allow me to elaborate a bit.
You will "feel" nothing. You'll have access to all RAM. However, Applications will also have to be able to utilize it (64-bit) otherwise you gain nothing AFAIK.
64-bit processes have full access to all CPU registers and functions, as opposed to the subset available to 32-bit processes. For this reason 64-bit programs written to perform calculations with very large numbers will be noticeably faster than the same programs compiled for 32-bit. Some BOINC
projects I used to run benefited greatly from this.
The 4GB limit in XP x86 applies to all addressable physical memory--including video card memory. The more memory you have on the video card, the less you'll have for the operating system and applications. This is the reason for the fluctuations in reported memory many see for XP x86 with 4GB of installed memory. If your video card has 512MB GDDR, you'll see 3.5GB of system RAM available in Windows. 1GB GDDR on the card... 3GB available RAM for Windows. Et cetera. XP x64 moves video RAM addresses beyond the 4GB border.
I'm less clear on how exactly XP x64 juggles RAM between 32-bit applications that allows those applications to work concurrently without bumping into each other's memory spaces. All I know is that it has to do with the WindowsOnWindows64 (WOW64) application compatibility layer; the addresses may be virtualized.
Some x86 might not work as x86 on x64 isn't fully compatible but those are rare exceptions (most of the problematic apps comes from those with drivers that doesn't exist for x64).
Hardware monitoring tools, like Speccy, must be specifically written for 64-bit operating systems to function. 16-bit applications will not function at all due to the fact that all 16-bit components have been removed from Microsoft's 64-bit opearting systems. This also means that rare 32-bit applications with 16-bit installers will fail to install.
Some drivers doesn't exist at all and you'll need a VM or equivalent.
I'd say most major hardware manufacturers have released 64-bit drivers since the release of Windows Vista, and began writing drivers to cover XP x64 around the same time. Smaller manufacturers for older hardware just couldn't justify the expense of rewriting drivers, I'm guessing. I've never had a problem finding drivers since SP2 was released in 2007. It pays to do your research before purchasing hardware.
Some dos apps/games won't work unless run in dosbox.
This is most likely related to the missing 16-bit Windows components. This is common for programs written prior to the release of Windows XP in 2001.
Your apps will be able to use natively all ram.
Only those 64-bit applications that are specifically written to take advantage of the extra memory will be allowed to use it. 32-bit applications will only see what they're used to seeing in a 32-bit operating system because of WOW64.
You'll get a little longer support (1 year) as Windows 2003 x64 is using the same binaries the hotfixes and security fixes will work on XP x64.
This is true, but it won't be officially supported. It remains to be seen whether update packages will continue to install as expected. The good news is that if Microsoft makes the expected changes to break functionality, a workaround has already proven to be effective
You'll begin the transition to x64 that will happen sooner or later.
The real transition began with the release of Windows Vista in 2006, when Microsoft began pushing hardware vendors to preinstall 64-bit OSes on new machines to be sold at retail. We're seven years in, and we're only now beginning to see the death of 32-bit operating systems. The mainstream transition from 16-bit to 32-bit beginning with Windows 95 took far less time.