Early pricing rumors point that 6850 would be ~200$.
OP, if you have to buy it today, just get GTX 460 1GB, maybe even OC version if you don't want to do that yourself. If you can wait few days, wait till 68xx. The good thing about ATI is CrossFire since it doesn't require an NVidia chipset or a hack to work (assuming you have CrossFire compatible mobo)