I've done this before. Get a frozen VGA cable for your Xbox, then get a KVM switch box, then mount to push buttons on the outside of the cab - one to power up the Xbox, the other to power the PC - just make sure you clearly label them up! Also, if you have a power strip, you can split the 'master' socket with a 2 way adaptor and whichever you power up (Xbox or PC), it will power up the entire cabinet just the same! It's easy stuff, just takes a little thought!

Personally, when I did it, I thought it was a nice feature to have the choice and it was good fun figuring it all out (which wasn't all that difficult or expensive to do!)