That would be enough to run this good budget card, granted its not ancient.
There are a few things you can do. You can get a new power supply and a GPU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371045&Tpk=Antec%20vp
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=7621659&CatId=3585
or you can just get a single GPU without having to change the power supply:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102969
The graphics cards have about the same amount of grunt. The GTS250 is an older unit, but still has the raw power to play most games. I have one paired with a budget Intel Celeron G530 and I am able to play Skyrim, Battlefiled Bad Company 2, and other games smoothly. It does lack DirectX11, yet it doesnt really have the power for DX11 anyway.
The 7750 is also a great card as well. It is roughly the same speed according to the chart. This card does support DX11, so newer games would look better. This card will also look better on the power bill, as it does not use much power.
You can even get a new case if you want, or if you have one, you can mount your computer in there, and have your own custom gaming computer made by HP.