Vista is case by case
I had the same issue this december and decided to get a new machine:
1) My Vidoea card was AGP - requiring a new Mother Board - which means a new system. A short term solution is add memory - 512 is very low and you should look at your swap file size and how the computer is managing itself (system restore points and actual programs running in the background. My previous computer was similar to your yours - I used a NVIDEA 6800 card and 1024 memory.
2) Vista will be the only supported OS in the near term - there is a follow on OS coming in 09 or early 10. So like it or not you will be operating Vista at some point. There are growing pains but having undergone XP prior to SP1 and SP2 I will say that this has not been as painful.
3) Vista has run everything very well - no glitches to date on SH III or IV. I suspect that the bulk of the issues are with upgrades or some conflict with Video/Sound and the game. I am currently running 2048 (Vista is a Memory hog) and plan on runing 4096.
Bottomline - I can send you the list of what I have but it is stable, easy to O/C and I am using a single 8800 GT video card - SHIII and SHIV load very quickly and run with zero issues or CTD.
Good luck
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