I just borrowed my friend’s Slingbox and tried its SlingPlayer for Mac, the performance is pleasing. You won’t have problem to get default smooth 320×240 video and great audio using broadband speed more than 300kps (My friend tried to connect to his Slingbox located in HK from US through 300kbp connection, and I tried with 500kps from a cafe in HK). If you don’t mind to scarify smoothness or allow a little bumping of voice, you can try 640×480, I guess if you connection can achieve around 700kps, it should be good for that.
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Playing
Playing in Dock