The G5 iMac is VESA-compatible, hoist away. I use a couple Logitech USB mice, 2 buttons plus scroll wheel, without anything like a driver install. Plug it in, wait one second, use it like it's always been there. The only thing your keyboard might not support is the CD-eject button.
I've never seen much value to the buyer for used machines. Buying a refurb direct from Apple is different. Exact same warranty, a couple bills cheaper. As you already have the 20" LCD and speed doesn't seem to be a deciding factor (i.e. 3D games, A/V intensive stuff), I'd go with the Mini myself. (Pooljam plays fine on my Mac as well)
I've never seen much value to the buyer for used machines. Buying a refurb direct from Apple is different. Exact same warranty, a couple bills cheaper. As you already have the 20" LCD and speed doesn't seem to be a deciding factor (i.e. 3D games, A/V intensive stuff), I'd go with the Mini myself. (Pooljam plays fine on my Mac as well)