MSI 790FX-GD70 - $120
AMD Sempron 140 - $35
2x1GB DDR3 - $FREE
4x HD 5830 - $450
Corsair AX850 850W - $170
+5% tax = $813.75
Let's say $850, some items have to be shipped.
@ 245 Mhash/s per card, that is 980 Mhash/s
Also electricity is 7.15 c/kwh, so 24/7 30 days per month @ 850W = $43.76/mo
1. Underclock the memory and overclock the gpu on your 5830's. The gpu should do ~900MHz without increasing voltage, which will yield a power consumption of about 160W and hash rate about 280Mhash/sec, per card.
2. undervolt and underclock your cpu. It should work normally at <1.5GHz and 1V. If you're gonna use linux or windows with the 2.1 sdk, the cpu will idle most of the time anyway, so it should use <10W.
3. if you use linux, 1GB of memory should be enough. That can save you another 5-10W.
4. again, if using linux, you can skip the hard drive and run it from a usb stick, saving another 10w
5. corsair's AX series is a bit overkill for this rig. A TX850W will do just as well and the few % efficiency difference doesn't justify the price difference. The TX costs about $50 less and the lower efficiency will cost you maybe like $2 per month.