Should just say it that everything is about the graphicmemory in GTA:IV sofar. If you have 256Mb you might get at a maximum 1280x1024, but then lacking in texture quality...
I myself have a 8800GTS 320Mb, at that cant go higher than 1440x900. I am used too play Farcry2, Crysis and other games att 1680x1050, but GTA won´t let me go higher than 1440x900.
So to sum up people who have for instance a 8600GT 1Gb, will be able to choice higher resolutions than 8800GTS320Mb, 8800GTS640MB, 8800GTX768MB.