it's all about the money. since you want to crossfire you should get either the 4890 or the 4770. which ever one you can afford.
That's a pretty big difference in performance.
My monitor is the acer x223 22inch 1680x1050
Some cards that you should be looking at then (in order of performance): 4770, 4850, 4870 512MB.
That ranges from about $100-$170.
However, your PSU is generic. If you would be going with anything above the 4770 you would most likely have to upgrade it or at least get dedicated power supply.
If the 4770 ever gets back in stock, that is what you are looking for.