The two graphicscards are identical from what I can see, the difference is the CPU and the case. Looks like one of them is out of stock now though.

edit: oh I see, one of them has 512 more megabytes of video RAM. You don't really need more than 512MB for Source games, though 1GB is really nice to have. It just gives you a little more.

I'd go with the bigger one.