Good to hear the switch to the Mac mini! I continue to be impressed by its success as more and more PC folks take notice.
As far as your network goes, I'm no expert, but I would start by trying to get everything on the same subnet (i.e. 10.0.0.XXX), including the wireless network.
Let me try to visualize your setup:
Router --- Mac mini
Mac mini ~~~ PC
And you want:
Mac mini ~~~ Xbox
I think you're trying to setup an "ad hoc" wireless network. I know this works in connecting a single wireless access point to another single wireless access point, but my guess is that you need a full-blown wireless router to do the job. I could be wrong if my network diagram is off, but that's what I'd try.