For your resolution then HD5670 is enough.
Also, OC your CPU a little bit (3.0GHz are great), that's should help a lot.
2.2 to 3.0 seems a bit hazardous, he could easily go 2.6GHz with no issues.
All of the money, you can go for a 5770.
Something that'll work great at that resolution : a 4850.
Cheaper and less performance then your previous card, but it'll run fine and let you save up for a larger monitor if you wanted.