Depends on the game. In cases where the bandwidth really matters and it bests even the 9800GTX+, then it might. But keep in mind that the 9800GTX+, aside from havign a 256-bit memory interface instead of a 384-bit one, is a hands-down superior card, having 64 texture address units to the 8800's 32, as well as significantly higher clock speeds.
And in that price range, for around $170US, the Radeon 4850 consistently edges out the 9800GTX+ by a 5-10% margin or so on average.
I'd go for a single 4870X2. It doesn't gain much of a benefit in CrossFire as of yet, and certainly not worth the money. A single 4870X2 would best a tri-SLi setup of 8800 ultras, and would, in most cases, not fall much behind a pair of GTX 280s, leaving them providing only a tad more performance for a good deal more cost, and a LOT more power consumption and heat production; you'll need a very strong and expensive power supply to support them, while you should be able to run the 4870X2 with your more typical (and affordable) mid-range power supply.
Quad 4870x2 just doesn't have the driver support right now. Quad 9800gx2 scales similarly when it first came out, and look at it now with mature drivers: http://www.guru3d.com/category/vga_2/
It scales relatively well for quad gpu across the board, in well scaling games like Call of Duty 4, it achieves near 100% scaling, almost doubling frame rates at some resolutions.
If history is any indication, this is a taste of how quad 4870x2 will perform after a similar amount of time. It's going to really shine, just not yet.
Get one 4870x2 now, and add another one later. Once drivers matures, you'll recognize the monster that quad 4870x2 setup is meant to be, instead of the cripple that it is now.