Right..
I was just saying that I am sure you could stick all the w35xx in this single-quad core mac pro and it would all run just fine.. the processors before the w3520 that is.. never tried it, and perhaps because they are so low in price I might experiement and let everyone know that the lower ones do in fact work.. but expect no turbo boost from the w3505, w3510. etc..
Xeon W3503 2400 MHz N/A 2 2 × 256 KB 4 MB 1 × 4.8 GT/s QPI 18× 3 × DDR3-1066 0.81.225 V 130 W LGA 1366 March 30, 2009
Xeon W3505 2533 MHz N/A 2 2 × 256 KB 4 MB 1 × 4.8 GT/s QPI 19× 3 × DDR3-1066 0.81.225 V 130 W LGA 1366 March 30, 2009
Quad Core
Xeon W3520 SLBEW (D0) 2667 MHz 1/1/1/2 4 4 × 256 KB 8 MB 1 × 4.8 GT/s QPI 20× 3 × DDR3-1066 1.225 V 130 W LGA 1366 March 30, 2009 AT80601000741AB $284
Xeon W3530 SLBKR (D0) 2800 MHz 1/1/1/2 4 4 × 256 KB 8 MB 1 × 4.8 GT/s QPI 21× 3 × DDR3-1066 1.225 V 130 W LGA 1366 AT80601000897AB
Xeon W3540 SLBEX (D0) 2933 MHz 1/1/1/2 4 4 × 256 KB 8 MB 1 × 4.8 GT/s QPI 22× 3 × DDR3-1066 1.225 V 130 W LGA 1366 March 30, 2009 AT80601000921AB $562
Xeon W3550 SLBEY (D0) 3067 MHz 1/1/1/2 4 4 × 256 KB 8 MB 1 × 4.8 GT/s QPI 23× 3 × DDR3-1066 1.225 V 130 W LGA 1366 August 9, 2009 AT80601002112AB $562
Xeon W3565 SLBEV (D0) 3200 MHz 1/1/1/2 4 4 × 256 KB 8 MB 1 × 4.8 GT/s QPI 24× 3 × DDR3-1066 0.801.225 V 130 W LGA 1366 November 1, 2009 AT80601002727AB $562
Xeon W3570 SLBES (D0) 3200 MHz 1/1/1/2 4 4 × 256 KB 8 MB 1 × 6.4 GT/s QPI 24× 3 × DDR3-1333 1.225 V 130 W LGA 1366 March 30, 2009 AT80601000918AB $999
Xeon W3580 SLBET (D0) 3333 MHz 1/1/1/2 4 4 × 256 KB 8 MB 1 × 6.4 GT/s QPI 25× 3 × DDR3-1333 1.225 V 130 W LGA 1366 August 9, 2009 AT80601002274AB $999
[edit]
It became available, and was the same price on the published Intel Quantity Price List. Now whether or not Apple got a discount or not, I don't know (expect so, but to a lesser degree than in the past, as I'm thinking they switched to Hon Hai for MP board production as well as system assembly).
But even if they did, I'd expect the W3580 was the same price as the W3570 anyway (same cost regardless).
BTW, the W3505 is a low cost part for a bare minumum workstation system (i.e. only need a dual core processor and ECC memory, which isn't possible on consumer parts at all). Think software that's not yet written (or can't be) for SMP (Symetric Multi-Processing), but still needs ECC protection for accurate results (i.e. recursive operations).