This will be a mining-only rig.
Would like to be in the 750M - 1 Ghash range
A budget helps

My friend and I put together a 1.2 GHps rig for about $1100. No case, no hdd/optical. <$1 per MHash is very doable.
5850s, AMD quad, free RAM, and MSI 890FX board with 1 kW PSU. Should work well. Building tonight =)
Lol, I put together a rig for <$500 that will put out almost 800Mhash/s.
Shop around is all I can say. Check lots of forums and websites for good deals.
Specs? Any used parts in the build?
My 2x 5850 rig will end up costing me:
$260 (Saphire 5850's)
$50 (Antec 900)
$0 (RAM already have)
$70 (750W Thermaltake Tough power 775W)
$45 Foxconn AMD motherboard with Dual PCI-E
$38 Sempron 140
$0 (laptop SATA drive)
So yea, less than $500 for hopefully ~700Mhash overclocked.