128bits has exactly half the data width of 256bit. Meaning this: 128bit has half the memory bandwidth of 256bit. That's very important to know. :-)
Yup. bandwidth basically is how wide the pipe your GPU has to suck data out of memory..
BUT there are some exceptions. so even though the 6600gt has 128bit memory, its still faster than the 9800pro (16GB/sec vs around 22GB/sec) in most situations.. unless from what Ive heard, your playing a game at 1600x1200 with 4xAA ... in which case both cards are usually too slow for that anyways. But yea, you DO want 256bit memory unless your specifically choosing between these 2 cards.