If you did not read a recent article on toms, the 512 frame buffer of the 9800gx2 can be an issue with high rez and high AA/AF.
That said, you are gonna pay the cost of another 8800GTX for the 9800gx2 anyway, so get another 8800GTX
Your power supply can dump **** loads of its total power into its 12 volt rails. so don't worry about that. Check my config my system takes 350 watts at full load. thats said yous should only be 100-150 more(less if you do not have as many hard drives as me).http://www.tomshardware.com/2008/03/18/nvidia_geforce_9800_gx2_review/