Mini-ITX based systems - worthwhile or not?

[pcAnywhere]

I would like to build a very simple PC to act as a music system / possible HTPC and was wondering about basing it on a mini-ITX mobo/cpu. These contain a 1GHz CPU so I'm (obviously) concerned about the lack of processer power if I do start to use the system to record / playback TV (I'm assuming that playing mp3 files is not going to tax even a slow processor!) My other option is to build a micro-ATX based system (using a cheap celery / semp) but I am rather taken with the ultra small format.

I'll be controling the system with a touchscreen monitor and using either 2.0 or 5.1 speaker systems (depending upon format / cost)