Hmm, graphics cards have Pixel Pipelines yes? These help with complex pictures and stuff and performance (I think, this is a partial n00b speaking). Now, the GTO2 seems to be basically a X850 PE (again, I think) but it has 4 of those pipelines locked ("turned off", normally has 16 pipelines, GTO2 only has 12 pipelines) and is underclocked. However, it is possible to unlock ("turn on") those 4 pipelines since they are still actually on the board, just disabled. Then you clock up the card up the card to x850 PE speeds and you got yourself a high-powered card for near half the price! It is also possible to just flash the X850 PE bios, but that can be dangerous and screw up the card <shrugs>
Again, this is from a partial n00b so I'm not absolutely certain this is correct