And the only time a person would notice the difference between DDR2 6000 and DDR2 6400 would be in benchmarks that are specifically testing the speed differences. In day to day operation, you are never going to notice. It might translate to a frame per second or two in a game, and as such, obviously if you can get it, grab this stuff, quick.
Honestly, the speed difference between even 800 and 667 is negligable, most people won't notice at all, especially since this stuff has better latency. If you're that worried about the speed, a faster hard drive would be way more noticable. Good price.