If you go second-hand, Pentium M pricing isn't too bad.
Pentium M 730 1.6GHz: £60-£70
Compatible Asus board: £40-£60
Asus CT-479 Adapter: £25
Then run
RM Clock, undervolt the chip, and set up Speedstep. That'll minimise power consumption when running idle, and automatically step-up when neccessary.
With this setup the hard disk will probably be consuming the most power, so consider a 5400RPM drive. Samsung do a nice 250GB 5400RPM drive.
I assume you're putting it in a cupboard for noise reasons? With components like these you can make a very, very quiet system so you'll be able to put it somewhere less enclosed with better airflow.
You can overclock the chip to at least 2.4GHz if needed in the future (e.g. for Vista but probably not necessary for a media server).
An ideal setup for a Media Center PC
IMO, if you want more grunt/dual core wait for the desktop Yonah boards - but they won't be cheap.