The 4670 compares to the 5570 (though it actually loses by a small margin). It should beat the 5550.
If I had to choose, I'd go for the 4670, or wait to buy something else. Yeah, the 5550 has DX11 ... but if you're really going to be doing anything that needs DX11, you'll want a better graphics card than that.
Go with the 4650 over the 5550. It's faster than the 5550. The only thing you're losing out on is DX11. But then again, if you were looking at DX11, you'd want a card better than the 5550. A 5770 perhaps?