Ditch the CPU, RAM, DVD, HDD.
Get a sempron 140 - uses about 50-60 less watts (if underclocked to 800mhz).  $35 shipped
Source free ram, dvd, hdd from other computers. $free
You can go with a cheaper mobo if you don't want latest features (i've seen some dual PCI-e for $59 shipped.
Upgrade later after you are done mining, parts will be cheaper in the future too.