x1600 pro should outperform the x700. go for pretty much the cheapest one there. from what I see it has he standard refference specs ATI sugested so there is no reason not to go for it. if you're tighter on cah the x700 i recomended is still somewhat compareable(again performance IS lesser and it lacks SM3.0[not a hug deal]) can't beleive I hadn't tried looking for the x1600pro...i thought of it... but... and BTK, those are x800GTOs... they just aren't priced THAT well...
the 512MB means that the card has 512MB of video memory on the card. It doesn't use your RAM. Besides 512MB is too much (unless you're rendering in CAD, or video editing, or something that takes up a lot of system resources).