Well The 4870x2 is certainly a good increase, as it is the top performer atm.
Don't think you PSU can keep with it, though.
Also the 4870x2 is a crossfire solution in a single card, so if there is no crossfire support, it will perform as a single 4870, wich is still a bump. Probably these days, crossfire and sli are good enough to not cause those problems, but it is still possible..
Other than that you have the 280 GTX which is the faster single chip atm. Other than that the 4870 1gb and the GTX 260 216 are next best thing.
Personally I would go with the 4870 over the GTX 260, but if money isnt the problem the GTX 280 tops those 2.
So, anyway you need a better PSU.
If your budget can go with a good PSU and a 4870x2, and you don't mind any problems a dual GPU can get go for it.
If not GTX 280> 4870 1gb> GTX 260 core 216.
I'm assume you planning on playing at 1920x1200 or higher.