Since the 2600XT core is a mid-low range one, its not going to be able to provide good FPS at above 1280x1024 resolution, no matter how much memory is has. And since the extra memory on the 512mb is not needed for this resolution on todays games, having 256mb of faster memory is probably the better choice. Also GDDR4 has lower power consumption and is supposably more overclockable.
I'd go for the GDDR4 version due to the higher memory speed and bandwidth over 512mb of slower GDDR3.
But then again the 512 is cheaper...
DDR4 version. It kills, man. I have one, and I managed 2300 in 3Dmark06 using Toms Hardware settings. Synthetic benchmark? Yes. Should the card be hitting 7900GTX level if it's not as powerful as the x850xt like tom's charts say? NO.
Drivers have made these cards great.
Oh, and I have the MSI version, and I use it at 1280x1024