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.