I would recommend going for a used 1st generation Xeon Processor (E, X, or L, such as X55xx or X56xx), such as a X5650 (A six core, can be bought for $40 used) If you look on Ebay, you should be able to get an older system like a Dell Precision T5500 (Dual Processor) or T3500 (Single Processor) or a HP Z400 (Single) or a Z600 (Dual Processor) These can be bought pretty cheap and still hold their value for workstations. I'm actually using a Z600 right now to type this.
If you are going for a gaming computer, I would recommend getting a processor with the 'w' prefix as these are the highest performance.
HP Z400:
Here
Dell T3500:
Here