There are some very cheap Xeon X3440/X3450 or better CPUs on ebay. They are overclockable, so it does not really matter which one you buy. The cheapest is the X3430, but it lacks hyperthreading, so don't buy that one.
The X3430 without OC. is approx. equivalent to an i5 4430. Pretty much enough for games, up to a 7970/GTX680.