I second the better but fewer option.
I think an ideal solution for the iMac, if you really must have surround, is something akin the Bose 3-2-1 system... It's virtual, but in the relatively closed environ of a desktop -- where you sit pretty close, and the "sweet spot" isn't that big -- you can get a surprisingly good illusion of surround. The rest of the time, if you're just milling about the room, you can play good old stereo.
I also like the iSub idea, though I'm not enamoured of USB Audio, with the bandwidth now on tap in USB2, it can work better... it might not fit now that they've really pared down the interface/clutter. Wires have almost disappeared. just a power cord, keyboard and short mouse cord.
iPod seems to be getting most of the mac specific 2.1 attention these days...
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