Since you have a SLI motherboard, I would try to go with Nvidia. They recently released their new GTX 400 series cards and they perform pretty well. The downside is they produce alot of heat and require alot of power, but the GTX 480 performs about on par with a GTX 295. They're kind of hard to come by right now but they'll probably be more in a few weeks.
If you never plan on getting a second card, then by all means go for the 5870.
It seems like the consensus is that a single HD 5870 is better than a two GPU set up. I'm guessing there's a cost-benefit analysis in there someplace. It might make sense to get an nVidia card to go with the nVidia mobo...
GTX4XX cards are virtually unavailable presently without a significant price-premium, which then drives their price/performance ratio into the ground.
First thing I'd do is call Velocity Micro and complain about the obvious mistake to see what they're willing to do to amend it. Missing something like this is truly a rookie system builder mistake.
Now, if you want to actually SLI 2 cards, find out what nVidia cards they have. They might have some GTS250's or GTX-series cards still, which would yield equal or greater performance than an actual CrossFire 4850 setup. Or, you could go with a single card, such as the 5870 you mentioned.
Whichever way you decide to go, I suggest holding out until they offer you some form of compensation for their error. If they say send the 4850's back and we'll send you a 5870 for $100, take the deal. If they offer up two GTS250's for SLI in exchange, that's a fair deal as well. Just don't let 'em burn ya, and if you're uncertain that what they offer is a fair deal, you have a source for opinions here at Tom's.
Velocity says they wrote "special drivers" that allows the mobo to run my cards in CF. If that's true it sure seems like they don't run in CF very well, given my experiences with games freezing up in that mode.
I'm leaning toward the single card solution. The Fermis look appealing though....I might take the 5870 for now and wait for the prices to come down on the others. Is there any compelling reason to go with two GTX 250s instead?