I'm with yrk1959 on this one. You may want to wait until Windows 7's release on 10/22 though. 7's media center is superior to Vista's. It was just announced at CEDIA that you can now roll your own, as far as CableCard tuners go. So you may be able to wipe that old XP box, buy yourself a CableCard tuner or two, install Win7, et voila! DVR, music management, videos, pictures, IPTV (well, sort of), DVD library management. As was mentioned, you can stream all of this to your xBox 360 - and through a pretty classy interface at that.
Here's the caveat: HTPCs are not for the faint of heart. While stability has improved over the years (I've been on the Media Center train since early 2004), these solutions are not remotely consumer electronics products yet. If you go this route, expect some troubleshooting in your future. Personally, I think the tradeoff is worth it - but that's just me.
If you hesitate to roll your own, and you don't want to go to the likes of Dell, there are numerous boutiques out there that build beautiful higher-end machines. Niveus, S1Digital, and Okoru come immediately to mind. Good luck!