The 5770 is your best bet. It's better then the 4850, (equal to a 4870). Costs less on your energy bill as it is very energy effceint, and it is a DirectX 11 part, which means you can have all the latest eye candy.
I personally have the 5770 in crossfire (runs as fast as a 5870 which is ~400 right now).
I originally upgraded from a 4850 to a 5770 for energy savings, but the upgrade itself raised my 3d mark score over 3k. It is a very good card. Maxs out most games without crossfire in 1080p resolution. The 5750 is less powerful, however if you crossfire them you get a 5850 for more then $100 less then the current cost of the 5850. All in all this generation is about crossfire and multi-gpu. I would say get the 5770 and when the 6XXXX series comes out you can trade it up.
Seems to me a 4850 + ram is more than enough with that resolution. 5770 would make it smoother than buttah, but if he's not getting a new monitor soon definitely get the 4850.
I'm personally getting a 5770 myself but that's because I'm buying a 22in 1080 monitor. If I was in the same situation, with my 1280x1024 monitor, 4850 and ram it would be. Oh, and if he really wants (and his mobo supports it) he could xfire then later on if he is having a graphics bottleneck, which I doubt.
Both would be very good for that resolution. I recommended the HD5770 because it will last longer and has DX11 and will serve well if/when the monitor is upgraded. Also it can be transferred to a new system by itself or in crossfire when the time comes. With the HD4850 you'll probably wanna ditch it entirely if you get a 1080p monitor or DX11 really takes off. It's a lot easier to just save up the $40 for the ram in the not too distant future.
BTW there is an HD5750 on Newegg now for $130; http://www.newegg.com/Product/Product.aspx?Item=N82E168...
So that plus the ram should be $170, just like the HD5770.