Well personally, If you can squeeze out under 200 dollars, I'd go for
[ame]http://www.amazon.com/Diamond-Viper-Radeon-512MB-GDDR4/dp/B000YIWESO[/ame]
the ATI HD 3870.. 512 Ram, GDDR4, supports DX10 if you need it, Shader Model 4.1... Or if you just need a fast solution.. something along the lines of an ATI X1650 Pro or X850 Pro.. the 1650 Has 512, can be picked up for 75 bucks or so, Runs the Exact same as the X850, which is more expensive, and has less RAM, I use the 1650, It runs Day of Defeat Source, TF2, all source games flawless, The only difference other than the RAM is the 850 does NOT support 3.0 shaders, and the 1650 Does, but it doesn't matter as source doesn't use 3.0 shaders anyway, so you could go with either and still be happy