I think two 4870s would suit perfectly here - or, like others have said, if you can wait, see what the 4870x2 is like.
Depends on your board though. Two 4870's are going to be crippled on a p35 and maybe still limited on a p45 so you need a x38/48 board if going 4870 in xfire so with the extra cost of a mobo, you might a well just get a 4870x2.