I don't know how you survive with just 1GB ram.
The graphics card need to fit in your case and be able to be reliably powered by your PSU - neither of which you give.
I gather you are currently using integrated graphics - from the Pentium.
All I can do is suggest a very low power card -
http://www.newegg.com/Product/ProductList.aspx?N=100007...
eg hd5450, GT 210