The 6600GT is the high-end card of the 6600 series. The core and memory are set to run "higher". Don't buy a 6600 256Mb if you had thought of that, they aren't just as good as 6600GT.
In fact, you will find that the amount of memory on the 6600 means nothing. The 6600GT will perform much better regardless of having less memory. Just a note. The memory of the 6600 runs at 500Mhz while the memory of the GT usually at 1000Mhz. So overall, the 6600GT would be a much better choice.