here are some prices from pricewatch com.
Balance your cpu with the graphic card.
The 6600 PCI express is the Biggest Bang for your buck. Price/Performance is outstanding. If you choose PCI express, then you will want 939 Pin cpu, and a matching motherboard capable of PCI express.
$482 - Athlon 64 4000+ 90nm Rev
$473 - Athlon 64 4000
$328 - Athlon 64 3800
$369 - Athlon 64 3800 512K 90nm Rev E
$334 - Athlon 64 3700+ 90nm Rev
$234 - Athlon 64 3700
$253 - Athlon 64 3500
$253 - Athlon 64 3500 939pin
$258 - Athlon 64 3500 90nm 939pin
$272 - Athlon 64 3500 512K 90nm Rev E
$161 - Athlon 64 3400
$143 - Athlon 64 3200
$169 - Athlon 64 3200 939pin
$189 - Athlon 64 3200 90nm 939pin
$198 - Athlon 64 3200 512K 90nm Rev E
$138 - Athlon 64 3000
$146 - Athlon 64 3000 939pin
$149 - Athlon 64 3000 90nm 939pin
$219 - GeForce 6600 GT PCI Express 256MB
$160 - GeForce 6600 GT PCI Express 128MB
$208 - GeForce 6600 GT OC 128MB
$161 - GeForce 6600 GT AGP 128MB
$137 - GeForce 6600 AGP 256MB
$129 - GeForce 6600 AGP 128MB