Without meaning to come across as a bit dismissive, if you get any replies to this thread they're going to be along the lines of 'I've got a 3870X2, it rocks, so buy the 3870X2', or 'I've got the GTX, it still rocks even though it's old, so buy the GTX/Ultra'.
There will also be those who say 'buy neither, and wait for the new 9XXX Nvidia parts'!
It's a bit of a recipe for a fanboy war, unfortunately.
This thread asked a similar question recently;
http://forums.overclockers.co.uk/showthread.php?t=17839155
And of course there is this huge 3870X2 thread as well;
http://forums.overclockers.co.uk/showthread.php?t=17835250
The best course of action I think would be take a good long look at all the reviews posted in the various threads recently about the 3870X2, decide what games you play/are most important to you, and then choose the one that gets the best results in those games.
Be aware of some driver issues with the 3870X2 and certain games, but also bear in mind these are being rapidly sorted, so it shouldn't be an issue for long.
They are both in the same price ballpark, although the OCUK 8800GTX looks good value for money, so it really is just a case of choosing the one that performs the best at the resolution/games that you play the most.