Nvidia cards are a little faster with openGL then ATi, but its a very little difference.
ATi cards are faster than Nvidia cards with DirectX, with noticeable difference.
do not get any Radeon with 256MB ram, get he 128MB. When buying the 256MB, its all marketing, there is no performance gain, unless you are doing CAD.
I would recommend getting the 9600 Pro, or if you can spend a little more money, 9500 Pro, or go all our and get hee 9800 Pro. But you should know that ATi disconnected the 9500 series (and 9700), and just anounced its disconnecting the 9600 Pro and Non-Pro. The 9600XT is taking its place.
going back to the 256MB vs 128MB, when buying the 256MB, they are using lower quality ram, like I said, its all just marketing gimmicks.