The amount of memory in Mb's is always a multiple of the GPUs Memory Bus Bandwidth.
The GTX 260/260 core 216/275 all have a 448 bit bus and, as such, it can use any amount of RAM in multiples of 448Mb.
The 8800GTX/Ultra has a 384 bit bus so it supports memory in multiples of 384Mb where as ATIs current top end cards all have a 256 bit bus and support memory in multiples of 256Mb.
OP, GTA4 is a very poorly coded console port.
Even so, when tested at 2560x1600 with identical settings, there was a negligible performance difference.
Looking at the Farcry 2 and Empire: Total War results, also at 2560x1600, the performance difference is also negligible.
The only real improvement the extra 1Gb had was enabling 8x AA in Empire: Total War.
As you are gaming at 1680x1050 and I doubt you are upgrading to a 30"+ ultra high res monitor any time soon, the extra RAM will do nothing for you other than drain your bank account faster.