Weird, you didn't gain much at all going to a quad....because your card is now overclocked which helped the most probably.
ExpensiveGet a new one lol. 4870 X2 is a power hog and a furnace to heat your house plus I had micro stutter in games when I tried one.
So I would pay $190 just for DX11 support? If I were going to buy a new video card I'd get one that had a significant performance improvementnot really. You could pick up a HD 5870 for $190 or so if i remember right. Should perform about the same, and is DX11. Then your open to CF later if you have the port for it.