Well I've been trying to make this decision for a some time already, and a gtx 570 is not as fast as 2 6870's, but it doesn't have all of those complications with multi GPUs. So you'd say its better to go for the stronger 570?
Well the decision is really up to you.I would want 6870's in a crossfire but i've heard of so many people having problems with it SLI/Crossfire not working properly,the worst being is that a game doesn't utlize SLI/Crossfire so you have 1 card thats at max and you have another card just sitting there and idling.
I say we get some more opinions before you make a final decsion.
Crossfire works pretty well for the most part. There are a few games where it doesn't work too well of course but I haven't had too many problems myself. Honestly the biggest concern is heat especially if the cards are close together with only one slot between them. I say just get a better single card now, and add another one latter.
I also advise getting a single card when possible. Some games can't use CF or SLI like GTA4. Other games have horrible scaling like WoW. (it's getting better with patches, but it still has a long way to go.) A single card won't have these issues.
If you research your games and know they handle CF well and are willing to risk any new ones that pop up will to, get the CF setup. The 6870s CF really well, and out perform two 5870s. There is no shame in getting a GTX570/6950 however.