RV775XT and PRO have 840 SP?

[renzokuken]

At the risk of sounding noobish (but in the spirit of learning!)...how important are SP's? I never understood how, for example, the GTX 280 has 240 SP's while the 4870 has 800 (!) and yet the 4870 isn't leaps and bounds better than the GTX 280 (in fact it's worse despite having more SP's and a higher GFLOPS output). Where does the "problem" lie in ATI's cards (the architecture?) and why can't we compare the performance between the two cards based on the # of SP's in ATI cards to the # of SP's in Nvidia cards? If anyone can answer either of these questions it would be greatly appreciated.