That'll make a bigger difference in a games performance than almost any choice of processor(short of buying a total crap CPU that's like $50). Of course there's a 99% chance you can overclock the other card by 100mhz and catch it up, but then again, the one that is faster to begin with can probably overclock as well to an even faster speed, if that's something you want to get in to.
All that being said, It's still a single-digit performance % difference, maybe 5-7% at best, but that's a bigger performance difference than many other choices you might make with a computer. Its a bigger difference than faster RAM, or overclocking your CPU from 3.4 to 4 ghz, etc.