Wow, that would be a sweet system. Although GTA:SA will only use 1 core, you should be able to convert video, run fraps recording, and play GTA:SA all at the same time without lagging your game at all . Oh yeah, that looks like it would run Crysis pretty well. You should consider setting up a RAID-0 to speed up that rig even more.
MTA Race is not supported by Windows Vista, you'll have to wait for MTA:DM or downgrade :/
RAID: http://en.wikipedia.org/wiki/RAID Basically, RAID-0 is two or more (identical) hard drives working together to boost performance by really increasing bandwidth. The drawback is that if 1 hard drive fails, the array of hard drives all fail, because the data is split between each hard drive. It should make Windows boot much faster and programs load up much faster too.
because its cheaper and, unless your running the top games at top quality (where the textures are insanely large) the 320mb gts actually runs faster then its big brother because of its increased clock speed.