If that is a 2.6 ghx single-core processor I HIGHLY doubt you have a PCI-Express slot on your motherboard. It sounds to me like you dont know much about comps (which isnt a bad thing). In order to build a good gaming computer, you need to have a bit of knowledge, especially when it comes to literally putting it together.
My recommendations:
4GB of MATCHED Ram (Maybe 6 if you want to run Vista w/ high quality games)
At least 500GB of hard drive space (to hold games! lol)
At least a 3.0 ghz dual core processor (chooose AMD or Intel)
Matching motherboard with at least 1 PCIX 16 slot
Video card in the 9000 series (I use NVIDIA) with at least 512mb of memory cache
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