just get the cheapest one you can find that isnt fully made out of plastic lol. iv had a few which are just sheet alu and have no actual structure/supports..really flimsy.
im thinking an E2xxx cpu would do ya nicely with a cheap 775 mobo cos ur not ocing. with as much of this as you can get ur hands on :D
http://www.costcentral.com/proddetail/HP/PX976AT/L49299/