So, it turns out that current versions of XBMC have their own uPNP server; so I don't really need to mess with MediaTomb (other than the fact that the apt repo evidently has a severely broken binary for MediaTomb) so I've got myself up and running.
I was going to work on getting LIRC working, but I can't for the life of me recall where my Hauppague or FireFly remotes are, eventhough I have the adapters in the machine lol... Plus I don't have my remote programming cable to just download a genaric code online right this second.
So for the time being I've got XBMC auto loading on the ubuntu box and I'm using the web ui as a remote (which I do quite enjoy actually).
It took a bit of time to get Apache configured to use the server as a local dev machine, but it's all up and running now.
I need to look into getting some more memory (I have 512x2+256); the motherboard supports 3gb, so that'd be nice... I'd also like to eventually add another drive to the machine as well.