about 10%-20% better depending on the benchmark, so about an average of 15%, which hardly justifies paying nearly twice as much, I just bought a Sapphire 4850 plus 2GBs OCZ DDR2 6400 for $154.99 after MIRs from the 'egg...
just click to add it to your cart and then click on "view all 14" under the combo deals, the RAM ends up basically being free after MIRs, you never know when you may need an extra 2GBs of spare RAM...
combo deal
so the price/performance champ (4850) and 2GBs of RAM for $154.99, while the 4870 costs almost $300...