I know others are recommending you get a load of tower units, but since you're a not-for-profit organisation, I'd guess you're a bit strapped for cash - unless you are lucky enough to get a nice big grant, of course!
I just thought I'd let you know that for the last year I've been editing bits and bobs using FCP 4.5 HD on a G4 mac mini! Although the editing I've been doing has just been simple edits, a few cross fades and no effects, it's been immensely reliable and has worked a treat. It's got me to the stage I wanted to be at - that I've used FCP and got to a basic knowledge of which does what. I've also been using an external hard drive to keep my media on.
The way I wanted to do it was get into the subject area without splashing loads of cash on a big expensive machine that I wouldn't appreciate because it was all I had known! I think there's something to be said for starting out on a shoestring - just a personal opinion, but it does make you value the more expensive equipment when you get it.
The other advantage of starting out small is that you can gauge the usefulness of the editing in your application - is it worth spending loads of cash when you're not quite sure how much it'll be used?
A G4 mac mini could be got off EBay for about £300, Final Cut Express for £150, a £120 external hard drive and you're off! Total cost under £600 for a complete editing solution.
Just my opinion...!
John