I have yet to find a game that works on XP that will NOT work on Vista 64-bit. Also, if you look at the benchmarking systems for games that the major review sites use (like hardocp, tomshardware, anandtech), all of them are Vista64.
Found one: Command and Conquer 3. Havent been able to get it to run past the loading screen. No biggie to me tho, Red Alert 3 will suffice for the time being.
anyways, regarding the OP, I have yet to run into a vista related problem while playing sins. To be honest, sins is not a demanding game regarding hardware. If any component it pushes somewhat hard is memory, specially on large, multi star maps. Also, i am running vista home premium 64 bit and love it. Hardware specs: quad core q9550 overclocked to 4.02, 4gig Corsair Dominator ddr2 ram and ATI HD4870 1gig and love the pretty graphics and eye candy w/o a stutter.
Might i make one suggestion as a seasoned computer hardware power user junkie, stick with 4gig of quality memory (Corsair dominator, OCZ, G Skill to name a few). Use the money thusly saved n look into a more substantial video card considering the 9800gt is literally a 8800gt that is overclocked. A fantastic budget card that will be a lil more future proof with upcoming games would be ATI 4870 512mb GDDR5 or even the 1gig version since they're both under 200 and are monsters. If you a nvidia kinda person
, the 260gtx / 260gtx core216 (the beefier 260 gtx) are of equivilent performance and price.
All in all, the 9800gt is a solid low cost card that does the job. Just know that if another game that ur eye is on of which is more graphically intense, you most likely will have to lower the settings to get some decent frame rates. This is all assuming your moniter resolution is 1680-1050, anything higher, the performance will be rough with most modern pc games such as crysis, Dawn of War II, Supreme Commander, GTA4 to name a few. If you have any questions, please feel free to send me a PM, ill be more than happy to help ya out.
Best of luck with ur new computer.