Those are two very different cards. Chose based on price, I would argue that they are roughly the same in terms of "bank for buck." The 7800 will have a longer lifespan.
Of those two cards.. the 7800GT, no questions about it..
The GTO2 is running at $279 right now.. with limited supply and last generation technology..
The 7800GT runs cooler, supports the latest technology, and is only a few more $ than the GTO2
the GT is the better bet
You have to OC and Unlock/Flash the GTO2 to get the bang for buck
GT is already banging at stock
GTO2 will need an aftermarket cooler to OC well
GT ownz with its single slot cooling