No, PSU rails do not add up that way, since the numbers given are for the maximum current on each rail, so if either rail is drawing it's maximum rated current, the other will draw the rest of the combined available current, which can be found by dividing the maximum combined power on the +12V rails (usually given) by 12. The combined current is what you're looking for.
And FYI, you can mix ATI cards with NVIDIA chipset boards and vice-versa without problems. THe HD 4850 is still superior to the 9800GTX and the Sapphire Toxic and HIS ICEQ4 editions come with third-party coolers already installed to facilitate lower temperatures and overclocking.