The 128 bit is the actual number of paths to the gpu per clock cycle. The more bits, the more memory bandwidth, the better theoretical performance. In reality this is nice but it doesn't have as much effect as the GPU's architecture and ability to process what is in the memory.
The higher the screen resolution the more memory a card needs. The amount of memory needed depends on the resolution and size of the connected monitor. Running your 15 inch monitor at 1600 x 1200 resolution is really cool but not really practical viewing. Also tests have shown that if you don't need the extra memory don't get it. 256 cards are a little slower at the lower resolutions.
I personally would not buy a FX5900 ultra, with the 6600 GT available for about the same price. There is a significantly better performance difference between the cards. The 6600 can run the latest games with very high quality settings at acceptable frame rates and the 5900U can not.
