7800GT hands down, VGA memory dont matter that much. What matters is speed, Pipelines, and technology. 7800GT excels in all of that, plus you can unlock pipelines to make it a GTX. If you have a choice between 6 series and 7 series, go for 7.
no, there is no GTX version of the 6000 range of Nvidia cards, only the 7000 rang. the 6800s have the 6800,6800GT and 6800Ultra, the 6800GT and Ultra have the same amount of piples but the ultra is overclocked and way over priced.
Sorry mate only the 7800GT can be unlocked and turned into a GTX.