The 850Pro has only 12 pipes, the rest are locked. To the question "how to unlock those damn pipes???", check this
The 850 does have higher stock clocks than the 800XT.. not much, but still. Stock performance should be almost identical.. but if you unlcok and overclock the 850 to XT or XT PE speeds, It would win by 5-10%.
I actually sat in your shoes a few weeks ago, but i found an 850 XT for a good price, so I got that instead. LOVE it. I finally can play fear on my 19" with (almost) all settings at max
well, just my €0.02 (€ naturally cos I live in Finland :P )