The Mini doesn't have the power to be a good HTPC IMO. A small Shuttle PC with a 300GB hard drive, a very fast processor, and two tv turners (one GPU/Tuner card) for about $1000. That's fully loaded. You could probably go dual core for about $150 more. Look for a Shuttle PC or SFF case, get yourself an ATI TV tuner/GPU and another tuner (HD if you like), then pick up the 300GB drive for $90 after rebate. You can get a Shuttle case that will take anything like an Athlon XP-M 2500, a Pentium M, A64 (dual core, Turion etc.), or a P4. If you get hardware accelerated tuners then you don't need much for a processor. You could get by with a P4 2.4 with HT and still be able to play high-definition content while capturing another show at the same time.
I just built a dual core P4 system for a friend that had one HDTV tuner, a 300GB hard drive, DX9 card, double layer DVD burner ($40), and a bunch more stuff for just about $900. It's ATX but the case he got was made specifically for HTPCs. Check HTPCnews.com for reviews on most of the components you might want.