If you crossfire a 6950 with a 6850, they will run at the same speed as a pair of 6850's - the cards have to run at the same frequency and with the same number of ROPs, etc to work in crossfire, so a higher end card can only use the same specs of the lower end card it is cross fired with (this is my understanding based on articles and talking with Sapphire tech support, I hope someone corrects me if I'm wrong)
Edit: My mistake, I re-checked the email from sapphire tech - you can crossfire a 6870 with a 6850 and it will run like a pair of 6850s (ie, you can cross two cards as long as they are the same series, 68xx, 69xx, but a 6870 and 6850 in crossfire operate the same as a pair of 6850s)
7850 is more comparable to the 6970, not the 6950 so it could be worth it for the $260 over $200, if you can get it good. But I have heard too much issues about 7850 drivers and they seem not fixed yet. But it could be worth it you know. Just make sure that 600W is a decent one.