I'm thinking of purchasing a XFX Radeon HD5870 for $100 off of Kijiji locally and would it severely bottleneck playing COD Modern Warfare2 and 3 on a 1680x1050 resolution on medium-max settings with my current AMD Phenom X3 8400 processor and 4 Gigs of DDR2 800Mhz of Ram? Thank you for all responses.
But the card should be fine for that resolution. Try it out, and if you feel the need later, upgrade to a PHII X4 or even an AthII X4. You might be surprised since the GPU does most of the work in games.