I am ditching my 21.5" iMac for the MP I just picked up:
$700: MP 2.66 2x2, 1 GB RAM, 250 GB HDD, SD
+
$90: 2x 1 TB Seagate Barracudas ($45 each)
+
$220: 2x Xeon e5320 quads (will do pin trick)
+
$100: Sapphire 5770 Vapor Edition (safe to flash this one)
+
$100: 2x2 GB FB-DIMM
$1000 less than the admittedly much faster refurb quad. It's a nice upgrade from a 3.06 C2D.
If I like the MP I'll get a SB version when they hit the refurb store. Knowing Macs I will still be able to sell my MP for a fair price.
Plus it should game at least as well as my notebook, which I can sell and use the funds to buy a 13" MBA.