Looking for a cheap upgrade on a graphics card for this older server

I'm looking to run Starcraft 2 smoother, but I'm not sure what to buy, I'm not trying to put too much into this pc, just for occasional play time, thank you in advance.

 

Without a budget I will just shoot out some thoughts to improve game play in SC2
1) get the system ram up to 4 GB
2) check your power supply, especially the connectors. You may not have the connections to power newer graphic cards. Then see how much you can support/afford from Tom's list:
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
3) Load times in SC2 can be annoying, go with an SSD if it bothers you.

 

The ram is up to 4 and it has a pci express slot ddr3 I believe

 

Still need to check out the power supply, all the high performance video cards pull more power than the PCIE bus can supply and require special connectors. Also the total power draw of the system needs to be calculated.
That being said, you can probably run an AMD R265 or GTX660 (need power connectors) and be happy as long as you are running 1080P or lower resolution. Your monitor shows 1024 so that is fine.
If you don't want to spend $150 there are cheaper options but your gameplay will suffer somewhat. Although the 4850 is "recommended" SC2 does not run all that well with that level of GPU, move up at least one tier on Tom's chart.

 

Ok I will do that, thank you very much.