Rolli is absolutely right--if you're buying new today, it's rare that a 2-card setup is your best choice.
As for the 7870, I assume this means you're looking around the high $200s as a price range? If you can afford it, the upgrade to a 7950 is substantial. There's a Gigabyte 7950 for $320 post-rebate here: http://www.newegg.com/Product/Product.aspx?Item=N82E16814125414
It seems like a good card, non-stock-cooler with good reviews overall. Compared to the 7870, the 7950 has 50% wider RAM bus with 50% more RAM, as well as 40% more shaders. It's a big upgrade and significantly more future-proof.
You could (should?) also consider waiting for the 660 Ti to come out. It may be a great card in its own right, but it may also change the market on 7870s and 7950s, since it's projected to release at $299.
edit: oh yeah, but the 7870 is still a good choice if you can't go over $300. Get the twin frozr for $260 post-rebate, still a good value: http://www.newegg.com/Product/Product.aspx?Item=N82E16814127662