Until this DX11/Win7 thing settles down, I'd advise you to keep the 8800s, you'll see no major improvements with a GTX275 and the superior 58xx cards are just not to be found or attracting a substantial price premium at the moment.
In some cases it would be a small benefit, in most cases it would actually be a little slower. Of course going with two GTX 275s would be a big upgrade, even though you would likely be CPU limited most of the time in that case. Anyway, if you wait for the mid 5xxx cards to be introduced you may want to get one and be ready for DX11. At the least it will drive down GTX275 prices to where you can get two. As it is I'm sure the 5850 will drive the price of the GTX285s even lower, when they have sufficient availability anyway.