I have it up and running on all my machines (Mac Pro, iMac x86, MacBook, G4 tower). Good to go so far.
So you have been busy the last days updating all your Mac boxes to Leopard.
Your iMac x86 is a first generation model with Intel Core cpu and with a mate display? Not the current Alu iMac?
Apple says that you need at least a G4 867Mhz for it run, so your 400Mhz machine will probably be quite slow.
Yes I can image that Leopard will not install or run quit slow on this old mac.
It's a quite old powermac 400 with PCI gfx that I use in the office for surfing, ssh shells, web radio, converting mac fonts, etc. No serious image/video/dtp applications. Maybe I should try to upgrade to tiger first. 10.3.9 is currently running on it.