The 9600 Pro you have linked is a fine card.. very good performer. It's the same one I was referring to at $68 but has gone up $2 off sale hehe.
The difference between 128mb and 256mb really doesn't come into play in this level of videocards- mainly because higher resolutions with antialiasing and super ultra textures is what consumes the most amount of vram. A 9600 Pro (or 9550 for that matter) doesn't have the GPU horsepower for any of this truly, so it's a moot point.
There is no difference in performance between a 128mb and 256mb otherwise. It's just how soon game texture settings, resolution and AA runs out of vram and has to start using system ram. In all actuality, a 9600 Pro will become fillrate limited long, long before it could effectively hit 128mb vram usage.
So I say go for the 128mb model. To give you some indication of the brackets of upgrade:
9600/9700 Series -Lower end
6600GT/X800 GT/9800XT - Middle end
X850/6800GT - High End
X1800/7800GTX - Ultra High End/cutting edge
I'd say not to worry about 128mb vs 256mb for anything than the High to Ultra End as these cards are the only ones where 128mb would start to get tight- since they can indeed handle playing games at 1600x1200 with 4-sample antialiasing and 100MB+ of textures in Ultra Game modes.
There are some rare exceptions- such as certain older NVIDIA cards where the speed of the memory used differred between the 128mb and 256mb models, so a smallish (like 2% hehe) difference could be yielded... but this had nothing to do with the memory size- just different/slightly higher clocked chips used on the physical board. The 9600 series doesn't have this to worry about.
