2 gts 250's in Sli perform close and sometimes beats a 5850 and a gtx 470is in the middle of a 5850 and 5870 so a gtx 470 should be better but not by that much.
But by price here in Australia you can get a gts 250 for $150 and a gtx470 goes for $440-50.So even 2 gts 250's in SLi are cheaper than a 5850. I don't know about where you live but thats a good deal here in aus.
Feature wise, a gtx 470 takes the cake mainly because of DX11
IMO I would go with the gtx 470 because I prefer 1 card solutions, some games don't scale well with 2 cards and because of heat and power issues but I was just stating the facts
here are the benchmarks for the gts 250's in SLIhttp://www.tomshardware.co.uk/charts/gaming-graphics-cards-charts-2009-high-quality/compare,1697.html?prod=on&prod=on&prod=on
Just in case' state your PSU