My best recommendation would be the ATI Radeon 4500
It's cheap and its really strong, you'll be surprised that it can run almost every game on full settings.
it's about 130-150$.
It's too bad that there never has been an ATI Radeon 4500. One of those mystical cards that never existed.
Now...there is the
ATI Radeon HD 4850 for about $150 which is a decent graphics card and will run most average games well. For your needs the much cheaper
Nvidia Geforce 8600GT (can be had for under $70) is a better choice.
Please guys, don't talk about GPUs and offer advice if you aren't aware of the market.