dual 9800 gtx's or dual 4850's

If you already have an SLI motherboard than go for the 9800 GTXs. If not then definately go for the 4850, because it is faster, especially with AA, and will perform much better in future titles.

Then I would go for the 4850 it about ties the 9800 GTX oc'd and kills it with AA enabled. Where the 4850 will really shine in this case is crossfire since it will scale better and in future titles, the 4850 will perform better than the 9800 GTX. Make sure to get an X38 or X48 motherboard since the 4850 takes a large hit in crossfire at anything below x16/x16. I would also recommend waiting a week or 2 to see if some custom coolers come out for these cards, I hear the 1GB version has a nice double slot cooler.

I would like to see some links to benchmarks.

I would go with the 4850s from the reviews I have seen.
They are generally as fast or faster depending on the tests.

And the tests they win they win by a decent margin and when the lose, they lose by a small amount.

Since the 4850 is a new generation card, their drivers are newer and games have not been fully patched to address possible performance issues with the new cards. As a result, performance is likely to increase more over time for teh 4850s than the 9800s

as others have already said.... definitely the hd 4850, though, also like others have said, I would wait for a dual-slot HD4850, that thing runs hot.

9800 gtx's, unless you don't care about having a PC / frying pan.

4850 x2 is faster!

I have 9800 GTX on SLI and I absolutely love it. 10,000 on 3D Mark vantage and no OC.

I run Age of Conan on max settings 1920x1200 on a 25.5" inch monitor. After a few hours of play, I'm at 59 celsius (high 40s low 50s at idle)

In sum, no complaints at all here.