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you can try looking for a quad-core '08 and dropping in a second processor later if you can't afford the 2.8 8-core straight up ($2000 or so). the standard model was an 8-core, so all the quad-core '08s have a second socket, unlike the '09-'10 models. they're hard to find, though.
 
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.
 
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