Re: Core stepping question
Yes the Antec 900 will fit the Tuniq Tower -- you just can't use the optional side panel fan.
I have no personal experience with overclocking quad cores.
From what I've seen others do, though, 3.6Ghz seems the norm for G0 Q6600, and B3's range from 3.2Ghz to 3.6Ghz. The big difference between the chips lies in their temperature output and tolerance -- the G0 runs cooler, and can tolerate more heat. The B3 runs hotter, and can tolerate less heat. So, even if you could overclock both chips to the same clock speed, the G0 would be running cooler, and probably a lot more stable.
MSI P6N Platinum SLI 650i
Cooler Master Stacker 830 SE
E2180 Core Duo @ 3.0Ghz (1.3vCore)+Stock HSF
2x 1GB G.skill @ 840Mhz 4-4-4-10-14
GeForce 8800 GTS 320 MB @ 655Mhz/1050Mhz
Samsung 19" SyncMaster 906BW