Well the x850XT PE is going for around 315$ on new egg.
It's a 45$ difference in price, and the performance gain aint that great compared how much more you pay.
Second, it's much more harder to find a x850XT PE at your local shop, than a 6800GT.
6800 series is pixel shader 3.0 compatible, making it more future proof, while x850XT doesnt (I dont want to get into debates whether 3.0 has any real advantage over 2.0b, but more and more develepers are using 3.0)
x850XT PE WILL be faster than a 6800GT...
So, it depends how much do you want to spend, and how long will the card be used...
Edit - if you can get the X800XL at that price, go for it! It's on par with 6800GT, in some games they are tied, in some it's somewhat slower, somewhere it's somewhat faster...
So if you can get it at that price, buy it!