around the GTX260 level, if you got the money go for the 4870X2. You could always pick up another one or even just another 4870, 4850, 4850X2 or 4830 card later.
Most the tests Ive seen (especially 3dmark test, and excluding Crysis which is Nvida bias) is that the 4870 is the single most powerful card even above the 280... But when you get into all the x2 type ones of course it loses out. Plus its pretty cheap too.
But it diffiently out performs the 260... might be on par on the 280 though depending on the tests.