To put it simply with this setup you have big problem as getting any decent graphic card will be impossible.
If you were to build decent gaming machine then it would require most likely to replace main board, case and power supply (ot to put it simply you would be building new computer and placing inside a few parts from old one).
Cost of it would be:
around 50$ for semi decent computer case:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119068
85$ for good class mobo with some oc abilities:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130095
50$ for good class power supply (430W will be enough for single graphic card setups)
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371006
115$ for radeon 4830 card :
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102803
cpu, ram, optical drives, fdd, keybord, mouse from old dell
then for 300$ it's decent pc gaming machine fully capable of playing anything in 1280x1024 (19" lcd native resolution) and as for sharp aquos it will be capable of producing playable framerates at games like mass effect , DMC 4, witcher in FULL HD for sharp aquos.
And if you have more money for 4850 or 4870 radeon graphic cards it will be even better.
The big dowside is you need help of computer savy friend to help you put it together.
PROUD MEMBER OF THE PSP RPG FAN CLUB
