The type of DDR is also important. There is SDRAM, DDR, DDR2 and DDR3. DDR3 A.K.A. GDDR3 is the best right now. When looking into buying a video card first I look at the core clock speed and the memory clock speed (keep in mind if it says 300MHz then it's 600MHz effective, it's always doubled). Then look at the pipelines, then the memory bit interface, then the Open GL and finally the amount of ram.
In this case, the 6600GT has a core clock of I'm assuming 500MHz and a memory clock of 500 or 1000MHz effective. The 6600 has a core clock of 300 and a memory clock of 500MHz or 1GHz effective. All of them should have 8 pipelines. They also all have a 128-bit memory interface. I think they all have Open GL 1.5 whereas some other 6600 and 6600GT's have Open GL 2.0. As for the ram, really there's no big deal. The 6600 doesn't have the technology to take full advantage of the extra ram.