I'm assuming you're talking about AGP. I've been debating the two myself the past week. The 6800gs is more expensive at $220, it comes new with a warranty and an increased possibility at unlocking the 4 extra pipelines. The X850Pro, depending on what card you get, is potentially unlockable to X850XT speeds. Considering you can purchase the AGP X850Pro on ebay for anywhere from $170-$200 shipped, it seems like this would be a great deal assuming you can unlock it of course (on par with the Microcenter deal a couple weeks ago).
My vote goees to the X850Pro just because of it's raw potential in not-too-modern PC games out there (which is primarily what i'd be playing with it). However, in reality it'd be hard to pass up the high assurability of the 6800gs. Unless I find a really good deal on E-bay -- and can verify the Serial Number of the card as one of the known unlockable batches -- I'll probably just wait it out and see what this 7800GS does to the market in a few weeks.
For your situation, buying a card for a friend, i'd definatly go with the 6800GS if you had to buy it right now. It's more reliable in my opinion.