What do you mean by "external box", exactly? A cable box / DVR, or an external infinitv box?
I use an infinitv card in my windows home server. I divide the 4 HD tuners that it provides between two different media center PC's. It works out very well. Easy to use, and more storage space than I know what to do with, and my cable card rental is only $4 per month from Verizon.
An external infinitv USB product would do the same thing, but one of my design goals was to minimize the amount of clutter there was below the displays. One box, one remote. Actually, by network bridging the tuners, I eliminated the need to attach the infinitv card to my HTPC at all.