HD 5770 if the power supply is reasonable
GTX 460 is also a good option
the Asus 768mb GTX 460 OCd with DirectCU cooling for about $140 after
mail in rebate is fantastic deal on Newegg if they are available to you
I think any card above a 5770 and you will start to bottleneck it like crazy.
What resoultion do you have?
The 6790 is coming out this week i suggest waiting until then as prices for the older 5xxx series cards will decrease in price.Also maybe the GTX460 too.It's supposed to cut right in between the 5770 and the 6870.
For your current resolution your current processor and card are a nice match. The HD4770 should still do fairly well at 1680x1050 but The HD5770 would be a nice choice or the HD5830 you mentioned if you are still spotting a good deal. The HD5770 is about 35% faster than the HD4770 at 1680x1050, the HD5830 is about 55% faster. It will be bottlenecked by your processor but this HD5850 for $140 is a steal and would serve you well if you go for a 1080p monitor instead or ever upgrade your processor; http://www.newegg.com/Product/Product.aspx?Item=N82E168...
It's a lil over twice as fast as your HD4770.
even for 1680x1050 i wouldnt go less than a gtx460 or 6850/5850. but that's because i like to have AA set to at least 4x and still get good fps. you absolutely MUST upgrade or overclock your CPU or you may not see much performance increase at all though.