I haven't heard great things about the codegen psus, apparently the 12v is prone to fluctuations so will make your system unreliable if you plugged in a 6800 requiring at least a good quality 350watt psu.
You can upgrade to a decent brand like antec for not that much if you look around.
Yes, 6800GT has the same power requirements, you're good. You may even be able to run an Ultra. Anyway, get the GT, best bang for the buck compared to the other new cards.