In most cases the 256MB Radeon 9600 Pro wins over a 256MB GeForceFX 5600 in PC games. The GeForce will have better compatibilty with emulators, and beats the 9600 non-pro most of the time.
If you want a nVidia card with comparable performance to the Radeon 9600 Pro and is not be expensive, it's the GeForceFX 5600 Ultra or 5700 Ultra.
In response to an earlier post, the 256MB GeForceFX 5600 is NOT a bad card! Don't make me post Dusk Ultra screenshots @ 8xAA + 8xAF! Oh, the GeForce 4200Ti does not support DX9 in hardware so no Dusk for you, Maxx77!