The 4870 X2 uses a little less power than two 4870s, and you will have the option to get another 4870 X2 latter on. The 4870 also has a sideport interface for additional bandwidth between the two GPU cores, but it doesn't seem like ATI has chosen to utilize it by enabling it in the drivers yet.
I thnk it's better to get 2 hd4870 in crossfire because they cost cheaper than one x2. But it is true that the x2 uses less power and it doesn't really outperform the crossfire cards. They perform the same. And if you lose one you'll always have another that performs great all by itself. And two cards in crossfire have less microstutter than x2 cards.
I have not seen a 4870 crossfire vs 4870x2 lately. I don't imaginethe difference is much, if anything.
The drivers are better for two 4870's, but run into problems in the board partner messes around with a custom bios. The two 4870's will also be cheeper.
It's really up to you. Now a days the performacne difference is nill. Get whichever you want, whichever saves money, whatever. You can't really go wrong. 2 Cards gives you spare carsd when you upgrade etc. A single card is more forgiving of your motherboard setup. Crossfire propper will have better drvier support in the coming years though, if you want to go that way. 4870's will be around as a 'budget' card long after the 5000's replace the x2 at the top of the line.
Thanks for your suggestions! Anyone else have an opinion? im still torn between which way to go. Another this is, Im going with the AM3 boards, and between the 3 im choosing, 2 of them have Support for 3 way crossfire, so if i went with 4870s in crossfire i could add a third later.