2.6 GHz Pentium 4, 512 MB RAM, 250 GB SATA HDD, Windows XP. The case varies, but it will be adequetly cooled.
I have built several computers before so I doubt I'll have any incompatibilities, but while looking for motherboards, I did specifically avoid ones with SiS-powered video, as well as trying to find one with AGP so I can add a dedicated video card: in this case, a Geforce 4 Ti4600.
edit: I still prefer a DVR, even if it will be slightly (but not excessively) more expensive as it will be more reliable, use less power, and be optimized to do its one purpose.