I don't think any of the benchmarks I've seen compare them directly, but you could also just look up specific games benchmarks for GT's in SLI vs the brand new 4870 reviews coming out today.
I would think the 4870 is the best buy if you're buying new; that way you can add a second one down the road during a big sale like Boxing week or something. You can also go with an Intel chipset that way!
and btw monsta. who cares if it runs hot (5 c higher than gtx card) its covered by a lifetime warranty so i dont think amd would offer that if there was a problem. You can always wait for partners to come out with 4850s with aftermarket coolers which will run incredibly cool and extremely overclockable