The 2900 Pro usually has more Memory (512). The 256 bit bus is not really a limiting factor if you compare it to the 512 bit pro. In addition there are many modded bios files and forum threads about overclocking it around.
I have seen some benchmarks showing the GTS(320) running out of memory which comes with a nasty FPS drop. I would go for the pro.
Just remember that the 2900 needs a lot of power.