If you're making a HTPC, I have no doubt you're somewhat concerned about noise, so I would stay away from anything Prescott (Like the Celeron-D) because they run so hot. (More heat means more/louder/faster fans, therefore, more noise.)
Performance-wise, I think the Celeron-D would be fine. It has 256K L2 cache (double the amount on the previous Celerons) and a 533 MHz FSB (up from 400 MHz), so I think it would not be as hideous a performer as the previous Celerons. I could see it being pretty competitive with a 533MHz Northwood P4 (which has 512K L2 cache) that's only one to three speed grades slower.
OTOH, I've never built an AMD system, so I'm probably not the best person to suggest an AMD alternative. (One thing I do know, however, is if I was building myself a new system today, from the ground up, it would have AMD inside.)
It's up to you, but if you opt for the Celeron-D, be sure to choose a case with good airflow (preferably, one that uses 120mm fans) and a good HSF. My current "no-brainer" recommendation for an effective but quiet HSF that's not too expensive is the Zalman 7000 Al/Cu. Stay far, far away from motherboards with northbridge fans, too, as they're completely unnecessary (especially at 533MHz FSB), and add a surprising volume of high-pitched noise to your computer.