I knew those 800 Stream processors were dying for more memory bandwidth. The 4870 has core clocks 20% higher than 4850, but the increased memory bandwidth lets it beat the 4850 by almost 30%. I think the true potential lies in the 1GB GDDR5 version, though, and it should be worth the wait and extra cost (within reason).
ATI may narrow the gap with GTX 280 to within 5%, and if they can manage to sell those 1GB cards for under $350, Nvidia will be in trouble.
I'm definitely rooting for ATI (we can finally build amazing systems without the drama, hassle, and extortion of SLI and the prerequisite Nforce chipsets) but hope Nvidia can at least keep it competitive in the long run. AMD/ATI can afford to play nice when they're the underdog, but if they come to dominate the market they're not immune to arrogance (remember when AMD's Athlon 64 destroyed Intel's processors, and people were paying $800 for FX processors?)
At any rate, I'm going to sit tight with my 4850 until the 1GB 4870 comes out, then I'll probably migrate the 4850 to a family computer so we'll have two PCs capable of playing UT3, COD4, and maybe even WIC maxed out if they can fix the driver.
BTW, I try not to miss an opportunity to remind people, but if you want to run two 48xx cards, the x8/x8 arrangement of P35 and P45 may present a bottleneck: