only problem with 260 is no DX11 support (no big deal) also Ati is normally more easy to get Xfire for most low and mid range boards dont have SLI but the have Xfire making upgrading later easier and an issue with 260 is places don't stock them anymore (pcccasegear doesn't) cause the 460 is pritty much a 260 with a DX11 update (much like the 8800gt/9800gt/gts250 what ever you want to call it card )
I was just putting it out there. The 5770/5830 would indeed be the best option due to Crossfire being piss easy to do and works like a charm most of the time (I personally never had any issues with a 4890 and 4870).
Get a 5770 now, then either get a second 5770 if you've currently got a Crossfire board - if not, then either upgrade the mobo along with it, or the entire system when you get around to it (in a year or so, etc).