Hm that really opens my eyes to what I thought would be worth it..
The differences aren't major, 10 fps being the most I think in a FPS difference. The only thing about OC'ing is it voids the warranty, so if it gets messed up by your OC or something random by chance you will have to fork up the money all over again, so in that sense spending the extra 40 bucks might be better in your eyes. I know that part speaks to me because I'd hate for something to go wrong down the line and have to replace it just because I OC'd.
I've been trying to weight the pros and cons for about a month now and its been a hard decision.
Heres a 7850 with a Core Clock at almost 1000.http://www.newegg.com/Product/Product.aspx?Item=N82E16814150660