http://www.extremetech.com/article2/...1780507,00.asp

An excellent article which will tell you which card gives you the best performance per $.
To give you a quick lowdown:
Best performance for fewest dollars spent (AGP)
1. Nvidia GeForce 6600GT 128MB AGP $201
2. ATI Radeon 9800 Pro 128MB $182

Among the higher end cards:
1. ATI Radeon X800 Pro $384
2. Nvidia GeForce 6800GT $372

Best performance for fewest dollars spent (PCIe)
1. GeForce 6600GT 128MB $190
2. Radeon X700 256MB $174

Higher end:
1. ATI Radeon X800 XL $339

If you want more detailed information, you'll have to read the article