It's a bit tricky to give definitive advice as everyone's needs will be slightly but significantly different.
The link above takes you to soundhousevectorlab where we detail the 2 machines we have in our labs for this purpose.
Some things to consider....
Celeron processors are not recommended for music and video work.
An 800 MHz Front side bus (FSB) is recommended.
1 Gig of Ram is recommended.
Again not celeron processor but Centrino and PentiumM are fine.
At least 512 Mb Ram recommended but again 1 Gig is better.
Having someone to ask about the inevitable teething and other questions is very valuable. So try to find a local friendly expert either at the place you purchase from or not. Its even worth being prepared to pay someone to take on this role if you have to - either on a 'on call' basis or for an initial period.
Hope this is useful...