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Get something like an EyeTV and hook the Wii to that. Going to the Cinema display directly will require some expensive adapters, best to go through the Mac first.
 
I've been thinking about the same thing lately, mainly because I have computer speakers that support Dolby Digital/ProLogic II, which my stereo does not. Therefore, I thought it would be nice to hook my 'Cube up to the computer and play through there, but it turns out that there is a small lag between DV cameras and BTV Pro/QuickTime/other capture app, so the games are unplayable. I've been investigating other ways, but what I really wish is that someone would make a consumer-grade PCIe video capture card. I'm not interested in spending thousands for a Kona 3 here.

On the other hand, if you have the cash, go for an Elgato EyeTV EZ, which is advertised as being good for playing games through your computer. I might check out the Hybrid mentioned earlier, though. It appears to be cheaper…
 
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