I have an older Pentium D 2.66 LGA775 just sitting around and i want to make it a set top box. The processor should be more than enough for a HTPC and an ATI card with the HDMI video and sound so getting the a/v to the TV is easy.

We recently have gotten digital cable with the HD package from east link with 2 receivers. One of which is HD PVR and the one I have to use with the HTPC is the standard SD one (Motorola DCT2524). My main concern is the HD content. Through the coax output on the SD box to my TV I can hear the HD content but I cannot see it since it's coming through the coax, which is fine but if I were to pop in a Tuner card, would I get the HD channels. Also, do I take the output from the box, or from the wall.

Any help would be greatly appreciated.

Well get rid of that SD receiver for starters heh. Can one get HD content through it?

Ya, you won't be able to get HD content through that older Motorola box for TV. If you're talking about "any HD Content" (since I'm not quite sure from your post) then by-pass the cable box when hooking it up to your TV and you should be good. If you do want HD TV through your HTPC however, say to record shows more permanently than having them stuck on your PVR, than you would have to have the HTPC hooked up to that HD Box.

Hope that helps.