4850 until you actually upgrade your monitor. Look at comparison reviews in actual games between DX9 and DX10, then DX10 and DX11 - there differences aren't very noticeable in most games. Some would argue that DX10 is worth it, but the next step is even less noticeable right now.
There is no such thing as "future ready", forget about that term it's made to trick fools - it rarely works out.
Think about your upgrade path:
If you have a good Crossfire Ready motherboard does it makes sense to go 5770 with your resolution and then add another - but still questionable depending on how fast the newer cards will be by then.
If your motherboard is not crossfire, then by the time you upgrade it you'll go for the latest generation or a single more powerful card anyway (5850 when the price drops, for example).
Consoles are anchoring the market and by the time a new generation is released you'll probably want to buy a new PC...
Save your money and wait until the Eyefinity setups become less expensive! That's my plan.
