well the new 8800GTS (512MB) is more powerful than the 8800GT and less powerful than the GTX and the Ultra(which is the best).Actually the 8600GT is better than the 8800GTS. So the 8600GT is only a small step below the 8800GTX.
i highly suggest geforce 8600GT for its superb performance and low price for about $179.99
Edit:
Specifications
GPU/VPU: NVIDIA GeForce 8600 GT
RAMDAC: Dual 400 MHz
Fill Rate per Second: 9 Billion pixels
Additional Features: Vista Ready
HDCP Enabled
HDTV Ready
SLI Ready
OpenGL 2.0
DirectX 10
Maximum Resolution: 2560 x 1600 (Digital)
Video Memory: 512MB
Memory Type: GDDR3
Core Clock: 565 MHz
Memory Clock: 1400 MHz
Shader Clock: 1242 MHz
Stream Processors: 32
Memory Bandwidth: 22.4GB/sec.
Interface Type: PCI Express
Interface Speed: 16x
Connector(s): Dual DVI (Dual Link)
HDTV/S-Video
Multiple Monitors Support: Yes