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.
With 4870 less than one month, I'd recommend to sell your card before its worth for price falls down further. You would probably get $100 now for it. Thereby spending $200 additional would put you on top of line with 4870.