It probably depends on your true needs. If XPx64 and all your current apps meet your needs for the foreseeable future, then I don't know that you need to be in a hurry to change. I assume we are talking about home use? IIRC, even after extended support was dropped for Win2K, updates continued to be released for some time for the truly critical problems that came up. And it is very likely that anti-malware apps will continue to be updated for XP, and by using them along with a hardware defense mechanism such as a good router, and of course being careful and aware when you are on line and having and using a good backup strategy, you really should be fine for quite a while. Since XP still enjoys such a large market share, there very well might be some third party that steps in with some kind of support program. I have seen rumors of such, but so far they are strictly rumors. And even if XP is not updated much, if at all anymore, the advantage to that is that no new problems will be created. New ones might be "found" but none will be "created". And as the majority of users shift further "upward" to Win7 and beyond, new exploits will be more and more focused in that direction as well. Some folks now say that Win98 is one of the "safest" OS out there since no one bothers to target it anymore.
If you feel you need, or want, to make a change, personally, I would suggest making every effort to see if you could get Win7 to run on your system, even if you need to make some hardware changes in order to do so, if you can afford it that is. It would seem likely to "future-proof" you better, since that seems to be a concern. I won't even tease you with suggesting Win8. I wouldn't do that to my worst enemy.
But I also do not really see a problem with Vista x64. It seems it would be the easiest change for you and would still give you official support for a while, and would give you a chance to get used to the newer ways that MS does things now, such as NTFS and WinSxS. Yes I know they seem more inefficient and bloated than you are used to, and they are from a hardware perspective. But look at it from a software creation cost perspective. People keep wanting their software to do more and more. In the old days, that meant having to tweak every bit of efficiency out of every byte of code, because, compared to today, memory and disc space were limited, CPU speed was slow, and all of those things were expensive. Now as hardware has improved, your smart phone is faster and has more storage space than desktops of old, and it's even "free", with a 2-year service plan. "Real" computers, ie desktops or laptops, are fast and have vast amounts of memory and storage space, for less money than the old systems cost, especially when you take inflation into account. What has gotten more expensive is what it costs to develop the software, ie manpower costs. And since the memory, disk space and CPU cycles are available, it is cheaper and quicker for the software developer to get more features by writing bloated, inefficient code, than to take the time it would require to write code like they used to write it. Also, some of those "inefficiencies", do provide more features, like the WinSxS folder. It gives you more abilities to uninstall updates that you wouldn't have otherwise, for example.
Again, because of the hardware improvements, there is just not the need for the normal user to do the kind of "liting" of their OS as there used to be. Even if any improvements in performance could be measured or even noticed, it just won't make enough of a difference to matter in normal day-to-day use. As an old timer that predates PCs, I remember using the early PCs that used 8" floppies or cassette tapes or even paper tape, and the 1MHz 8-bit CPU ran its OS and apps in a total of 64K bytes of total memory, at most, so it is horrifying to see the amount of memory and disc space that OS and apps take up today. But when I wrote code "back in the day", we would spend days optimizing routines to save 10 bytes and 6 machine cycles because it mattered back then. Now it doesn't.
If you choose to try Vista or Win7, I would suggest not trying to "lite" the OS at all at first. In fact I would suggest installing it with every option enabled and live with it a while. Try all the features and options. Absolutely tweak and enhance it however you want, but don't remove or disable anything at first. (This will also ensure the most compatibility with other software and cause less problems with "missing" features or future updates.) Once you have seen what you truly don't need or don't like or doesn't meet your particular needs with your other software or whatever, then you can begin disabling features. Continue to live with it like that for a while so that if you were wrong, then it's easy to re-enable the feature and you're back in business. Only after you are really, really sure you don't need or want something should you even consider removing it, and even then, will it really make a difference? If not, just leave it disabled.
Sorry for getting off topic. I know all of these suggestions aren't really appropriate to someone who has used computers for as long as I'm sure you have, but you got me started and it all seemed to be related. heh-heh
Anyway, I don't know if anything I said will make any difference to you, but good luck with your decision.
Cheers and Regards