first HTPC - Linux Hardware ?

I've followed and loved this forum since ripping and burning my first movie eons ago.

Now I'm finally ready to move from my current setup (my "main" XP Pro machine streaming to my xbox360) to a more / less full blown / dedicated HTPC.

I've been trying to get a handle on Linux v. Windows debates around here, and leaning toward Linux to a) avoid DRM & b) use older hardware (?)

I'd like some help as right now my biggest complaint is that my XP (Media Center) machine and xbox360 often don't let me stream HD videos.

I have a few old PC boxes that I can quickly salvage and add tuner/video cards as needed.

My first Question,
- should (can) I go the way of the Linux "media server" sending HD video to the Xbox360? or buy a old xbox to run XBMC?

Second, if I go that route do I need a "good" video card in the Linux server?

Third, my issue with the "old" machine for Linux is the lack of a PCIe slot, thus I would need an AGP or PCI video card and those are becoming costly vs. top end PCIe cards. and an upgraded Mobo means CPU as well...i.e. I'm wondering if I can truly do this using "spare parts"

Thanks in advance for any help!!!

That's kind of why I ask, the 360 "normally" needs a Windows Media center share to stream content, and even then there are a lot of formats that it will not play/transcode.

I'm reading about uShare, but not sure if that will allow me to use the xbox360 as I intended