It's the RAM folks, bring up your RAM usage before and while the game is running.
Vista on IDLE eats up over 1 gig of RAM just to sit there and look pretty, it's awful (I know, I'm using Vista 64 with 4 gigs of RAM).
After I kick up SH4 I regularly watch my ram usage go into 60%, running stock 1.4 game. Granted, I have EVERYTHING turned to max, and the game never stutters, but I can't even imagine running SH4 with 2 gigs of RAM...
During my initial set up (Vista 64 won't boot if you have more then 3 gigs, I think, theres a stupid hotfix I needed to apply first), just running Vista on 2 gigs wasn't pleasant...running a graphics heavy game on it will probably be pretty slow.
It'll run, but you'll need to turn as much off in the bells department as possible, and it'll probably still chug in places, but that's because the OS is eating half your RAM just to sit there
A good GPU will help obviously.