$153 for a 256MB 6800? We have a winner.
As for the X600, does no one bother to read Anandtech's reviews anymore? Surely they put out an article or three on that card. Heck, I'm sure it was included in their HL2 midrange roundup.
Short answer, though, is that a X600 should be basically twice as fast as a X300SE, simply because it has twice the memory bandwidth (128- vs. 64-bit path to memory)