You got a big bargain there spub, for gaming on a PC you need a quad core and the best graphic card you can afford
not really true as next to no game developers write games to use dual core cpus fully yet (crysis being the only one that springs to mind), a quad is overkill and will be for a year or so yet.
smack on with the graphics card though, currently the nvidia 8800gts is the one to own, the 512 ram version.
but as Naz said pc gming is a fast moving business and your gonna want a rig you can overclock, or your gonna need to stay upto date on graphics cards at least.
my xbl name is roneizm, although i do all my gaming on the pc and never use xbl