Hi, I currently run an Ati 5850, and I love it! I can run every game, most of which I can totally max out.
Isn't this ^^^^^^^ enough of an answer for you?
Honestly, I was considering the 5850 crossfire too, but I'm so happy with the performance now, and nothing is really stressing it at all (OH NO I CAN'T PLAY CRYSIS WITH MORE THAN 4X AA MAXED OUT) so i decided to wait. If you're fine with performance now, I'd at least wait until performance is notably bothering you. Remember, this is my opinion though, and its your money.
If you do want to squeeze more performance out of it, have you overclocked your card yet?
BTW, what resolution are you running at?