Building your own is the way to go since you are interested in gaming. You can build your machine to be compatible and supportive of your Graphics card. If you have not built a P.C. yet, the physical act of assembly is fairly straight forward and fun. Do a bunch of research on the different components that you will be using, make sure they work as good as possible 'together'. Be careful of ESD. Have fun.
Grand Total = £377.78 and shipping is free on all the items. Using the HSF I've recommended, you can OC the processor to get Core 2 Duo performance from it. All other parts are of excellent quality and will work together perfectly. I hope I helped.
Oh, it was no problem mate.
As for the CPU upgrade, it'd depend on what you can afford. But the dual-core price vs. performance king would be the E6750.
And yes, 64-bit Vista will run fine with that system, no problems.