I was looking at this myself,to play some older games on that hate new hardware.
Intel Atom Motherboard/VGA combohttp://www.newegg.com/Product/Product.aspx?Item=N82E16813121342
ITX formfactor case/power supplyhttp://www.newegg.com/Product/Product.aspx?Item=N82E16811154084
2 gigs of RAMhttp://www.newegg.com/Product/Product.aspx?Item=N82E16820141099
thats the base system,of course you would need a hardrive (I would suggest getting a 2.5'' laptop HDD to have more room in that small case),an optical drive maybe,and you might need a PCI video card if the onboard cant handle what your doing.
But I built a system for $250 (not really,just threw stuff in my shopping cart),that was with a DVD drive and a ATI 9250 PCI video card. I already have an 80 gig laptop HDD laying around