GT means more pipelines. (I am tempted to say MB signifies Megabyte). The increased onboard vid ram provides space for storage of texture and other maps. But lesser ram don't mean low quality maps since system ram can be used. Anyway only a very few high end games are utilizing more than 128Mb.
But pipelines, on the other hand, cannot be compensated by better cpu or system ram. Pipelines mean more vertices and pixels are processed per clock of gpu.
Another important diference is that the gt version runs at 500MHz core and 1000MHx mem while the non-gts run at 300MHz core and 500MHz mem. Big dfference.
Finally, the GT version's mem is a ddr3 while the non-gt is just a ddr.
So, GT wins over MB, since there are no 6600gt 256mb version.