There are a few differences between the two models (larger cache, twice the VRAM, .1Ghz faster) but the only place you might notice a difference is in newer more demanding games (due to the higher amount of VRAM). If you're just playing Half-Life 2 or something older like that, you won't notice any difference at all, but if you play a more demanding game like Oblivion or Crysis (via bootcamp) you'll notice slightly better performance.
In a nutshell, it's not really worth the $300 unless you want to push every last ounce of power out of your games. You'll be happy either way though!
Trent out