redgarl :
I don't recommend the 5770, I think it<s price isn't really well placed for a mere 4870 with DX11 support'
If you can find a 4890 for 170$, just go for it. There is no DX11 games right now and we are yet to see a game fully native in DX11... I don't think it will happens any soon since all the PC games are now console ports.
To the OP: This is an important point to consider, by which I mean, the decision to invest in a DX11 card depends a lot on what your current CPU/motherboard is.
If you've got a Q9550 or some other powerful quad core, you'll probably be keeping that system for a few years, so a 5xxx card supporting DX should be a wise investment.
On the other hand, if you've got an early dual core machine, you'll probably build a new system anyway before they go to DX11-only games. So a 48xx card will be fine.
Also, if you're in the latter situation (which I'm guessing is more likely, based on the fact that a 256MB card has apparently been through a full life cycle with your machine), a 4890 may be overkill. To take full advantage of the power of a 4890 or even a 4870, you probably want at least a mid-to-late generation Core 2 or an early quad core CPU; otherwise your CPU will likely limit performance well before the card. Depending on how old your CPU is, you might even max out your performance with a 4850 or 4770.