a single 4850 performs about as well as a 9800 GTX+, though pulls ahead when you use AA
I'd still get a 4850, because they use less power, are cheaper, and have more overclocking potential, as well as having a single slot cooler.
G92 is a good GPU, but is already being pushed to its limits with 9800 GTX+
the only thing is, the 4850 tends to run a bit hot. The GPU is designed to handle a higher amount of heat though, and the fan speed, by default, is not that high (which you can increase and have the temperatures drop significantly)
*edit*
If you get a second 8800 GTS 640MB card, it will use a lot more power. the 8800 GTS 320/640, GTX and Ultra's all use the G80 GPU, which uses a lot more power than G92