What mobo have you got at present? If its a decent one then simply picking up a Q6600 and clocking it would be your best bet, however if its not a very good board youd be best going AMD route, you could pick up an AM2+ board to allow you to re-use your existing RAM and add in a Phenom x4 955, or alternatively pick up an AM3 board, 4 gig DDR3 and the same CPU if it fits in budget or if not an Athlon x4 630