Jesus, that's a hard question. Seen as your going to be using this comp mainly for gaming, then I would suggest getting the 7800GTX with the 3700 San Diego purely because there aren't that many games out at the minute that support dual core technology.
But to make it more future-proof, I would go with the X2. Damn, that is a hard question, but I think most people on this forum would recommend the 7800GTX with the 3700.
It's up to you though. Have you thought about going down to a 7800GT? I mean the extra 4 pipelines aren't THAT much to obsessed over, and I'm sure someone in the near future will learn how to unlock them. If you do go down to a 7800GT, can you afford an X2 CPU?