Just get an old OEM prebuilt on the sandy bridge architecture. Like an i5-2500 or i7-2600. They are all over ebay, refubished, for only 150 bucks or less. Wack a decent GPU in and you have a decent mining rig. Just don't put in a really high performance card in or it will bottle neck. Also, might be a good idea to upgrade their power supplies as they are normally quite low wattage. Around 320 -350. Not enough for a decent card.