I've been trying my Intel iMac (typing on it now) for a few days and I haven't noticed a big difference in performance compared to my PowerMac. I'll try out Mountain Lion on the iMac, as many of you are saying it sped it up.
Last year I've bought a bunch of white c2duo iMacs for my work mainly to be used for RemoteDesktop with a Win2008Server. They all run Lion with Office08, Safari, FireFox, email as my most used progs beside the RDP-function.
Web, YouTube and video run seamless too just out of the box and I very like the good audio-performance.
Standby is on low power-consumption and I guess both on full workload and stand-by the power-consumption of the iMac's is far below the PowerMac's.
I would run the iMac as my main machine on low-power costs and fire-up the G5 on demand for any special tasks using ScreenSharing and also the additional monitor to control the G5 via iMac.
It's a pity, your iMacs screen is broken - I'd even go for a "new" white iMac for that purpose, cause I like it's design ...
As for my white iMacs and my demands at work I'm pretty content with the overall performance of Lion.
But I'd also like to know, how to install any OS X beyond Lion, since the white iMacs suffer from that limiting 32-bit EFI-Rom.