Ordered a Dell 
On Jody's advice, I ordered a Dell Dimension 5150.
- Intel Pentium-D 3.0GHz 930 CPU
- 4x512MB RAM
- 1x250GB disk
- Twin optical drives
It also comes with a 19" flat panel, which will be nice to have. Turns out to be more expensive than parting up was, but not excessively so.
The only major thing I am giving up is the second disk, and we can probably put that in ourselves at a later date. The CPU speed drop shouldn't be a killer (Dell wanted !
$400 more for the 940 so f!! that). I'm not so sure about the RAM, however I seem to recall that Dells use their own Dell-specific RAM factor so I would be stuck with Dell for upgrades anyways.
Delivery should be in the next 5 to 10 days.