I'm in a similar situation as OP. Have been using latest MacBook Airs since 2008 and really got used to SSD disks.
Now I'll be using a iMac G4 1.25 GHz as only machine. Just made a bet that I'll keep it as my only computer for the coming 5 years. Upgraded RAM to 2GB and space to 400GB. Running
TenFourFox as browser and it works pretty ok, really nice to have an updated Firefox browser. Office 2008 is painfully slow so I'm sticking with 2004. Adobe CS4 is not very different compared to CS5 in my uses. Transmission and VLC works fine. With Leopard it syncs my iPhone fine.