You will have to have a dual processor so as to not bottle neck your GFX Card.
I have a 7800gtx 256mb. I have had it for a while now. 2 months ago i had a p4 2.8 cpu... i was running Test Drive Unlimited and the frame rates weren't too high ... I've then updated to a Intel E6550 Duo and now it runs like a dream
I think of Vista as a nice drink... you have loads of it while you are out and love it.. but then in the morning you wake up and your have a mean hangover....
Your CPU is fine, I would not even think about upgrading that for at least another year to two years. Vista is only something that should be upgraded to if, you want the added features and want the more advanced user interface. It is by no means a necessary upgrade, just a nice upgrade that is good if you can afford it. The performance you will get in vista will be the same if not a little bit lower than WindowsXP so there is no need if you’re a gamer.
But saying that Vista64 is good if you want some better graphics and have a lot of memory in your system to use. Despite what some people are saying I can notice the difference when playing Crysis in DX10 over playing it in DX9 with the tweaks. But to be honest I think it’s a situation where if you have never played the DX10 version and just use the tweaked DX9 version you will never miss what you have never had.
You're gonna want at least an x1950 or 7900 to play crysis on ok settings I would say, but I can't really reccomend you get one of those when the 8800GT is available for around 60 or 70 quid more with twice or more the performance.