Well, I know it can be unlocked, but it's not a hundred percent. May have artifacting, or other glitches. Plus you lose 128mb of video RAM from the GT version. So it's not really a GT, but close.
At stock speeds, the X800GTO is faster, but I'd rather have four extra pipes then 128MB of RAM any day of the week since you can do this with a fully unlocked 6800NU. Pipes are more important to horsepower then memory.
As for unlocking, taking any card out of specs is a crap-shoot. From what I know, there is a high success rate with the 6800NU... ergo the praise of this card... also from what I know, I haven't heard any such stories from "lesser" 6800 cards like the LE or XT (though they may exist).
Bottom line, if you're going to buy the card and stick with it at out-of-the-box specs, go with the X800GTO... if you're going to take full advantage of the silicon and are willing to take a slight gamble, go with the 6800NU... they have had a HIGH success rate at unlocking without problems.
To be honest, I avoided this entire price-range of cards... it was all too confusing... I just ponied-up for a 6800GT about a month ago... I'm happy, and I love CoD2 on it. Good luck.