you could get two 120GB hard drives if you did not want to buy a SATA card and do a Striped RAID so you have a single 240GB drive.

What I did with my quicksilver G4(because I was young and poor at the time) is take out both the zip and dvd drive replaced them with PATA drives instead. From memory you have to assign the hard drives jumpers, the zip is Slave and the dvd drive is master.

When I wanted to upgrade the g4 to leopard, I cleared off a hard drive and cloned an OS X iso to the drive and booted from that.

Of course, this is banking on you not need an optical drive, or if you have an external instead.