The cheapest options that I can think of is getting a celeron2 566 and overclock it to 850, which would cost you last then 120 or so, just paying for the cpu and a slotkey.
If this is not enough for you, might try going with a p3-600 to 700e and oc that, those are around 200 or so and grab a new motherboard to go along with that if you want.
my 2c