Oh yeah, it will work. I believe that board runs both slots at 16x so there shouldn't be a noticeable performance difference from not using PCI-E 2.0. Two 4850s would provide you with slightly better performance and be a little cheaper, at the cost of using a bit more power of course. Anyway just make sure your PSU can handle whatever GPU(s) you upgrade to.
Yeah but he could get a 4870 now and a 4870 later. Buying two for crossfire right off the bat is just going to put you behind in money. Next time you want to upgrade you'll have to buy a newer card (more expensive). Whereas buying a 4870/4890 now, then buying another a year from now when it will be even cheaper = cheaper overall.
If both ports run in 8x...that will bottleneck your performance somewhat when combined with the 2.0 cards in the 1.0 slot. Just another reason to buy a single card in my opinion.
Agree with Fallen on this one. better bet will be the go for single 4870/4890 now and then another single in about 6 months when they are dirt cheap do to 5000 series as well as price drops to the new cards.
The prices will go down soon on the 48XX cards and then when prices come down on the 58XX it will come down again on the older ones.