Really dumb question but can you setup OSX to have both a 10.4 and 10.5 partition? Or am I better off just getting an external drive and booting off that for 10.4 testing?

I heard people were dual-booting between the two when Leopard was still in beta. You might be able to partition your disk in disk utility, but I myself have never done it before so just make sure you back everything up.