Not sure I would go for one of those mini Macs they would suffer the same as the iMacs from any modifications being impossible. I fairly recently bought a twin G4 500MHz with 1GB RAM, twin HDD and all the usual Mac extras and a 17" flat screen CRT for $1100 as an ex leased machine. Works fantastically
Depending on what you want the machine to do, and if you are mainly going to work in Unix, then maybe a G4 may be the way to go - but get a tower unit so you can modify/upgrade it!!
Cheers