We just had a similar
thread. The chipsets discussed in the other thread are available in laptops and all-in-one
motherboards. Note that this was supported by Current.
Combine this with your old monitor, sata hard drive +/- dvdrw, power supply (can your old case be used?) and you would get away for under $300USD.
I feel all-in-one motherboards tend to be inexpensive, compared to buying a separate cpu, graphic card, etc, and offer reasonable, energy efficient performance.