If I recall the B&W g3's stock pwer supply is nothing more than a 220 watt atx psu anyway, so you should be set.

-Jeff
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Mobile-2.2Ghtz Core 2 Macbook, 4GB ram, 160GB HDD
Server-Dual 1.42 MDD G4, 1GB ram, 4 random hard drives.
PC-C2D 2.4Ghtz, Asus Commando, 2Gb ram, 3 hdd's in raid 5, 8800GTS.