i could play it with a 2.7ghz celeron and a 9800 pro with a 512mb pc 2700 stick of ram. the cheapest solution that you can do is (and that still leaves plenty of room for powerful games) is do what i did.
AMD Athlon64 3000+
MSI NX6600GT pci-ex
MSI Neo 4 Platinum SLI mobo
and make sure you have a 500w power supply so you can handle dual 6600gt's and your set. i get 7682 in 3dmark03 and the pc2700 ram is slowing it down. also youll get over 100fps on CSS with high settings. or are you going to upgrade your computer you have right there? what are you thining of upgrading