My botting computer has an Intel core duo E5200 processor, 4gigs of ram (kingston 800mhz) in dual chan configuration, win 7 pro 64bit, an ATI HD 3850 1gb ram and the motherboards is a FoxConn g33m02 pulled out from a Dell Vostro 400 (very crappy imo).
I manage to run 5 GB bots and 1 hb bot with about 80% cpu use (HB requires more cpu time).
I always use this computer remotely, through Tightvnc or Logmein (both require some cpu time aswell).
I bought the CPU+cooler, motherboard, video card, power supply and 2 gigs of ram (I added some more after a few weeks) for 170€ (ofcourse they all were used) about 2 months ago. Compared to market prices in Italy, it was a fair good price.
If you want to save money just buy an used computer. DON'T save on motherboard and power supply, here's my story:
After some weeks a capacitor into the power supply just exploded (power supply is crappy), and i had to replace it, and the fan was making a lot of noise and i had to replace it aswell. I wanted to add some ram,so i took two no-brand ram with elpida chips, but my (crappy) motherboard didn't like them, so I had to send them back and get a better brand (it costed me time, money and a lot of swears).
PS: I use this computer just to bot!