I used to have a Dual 2GHz G5 (the 970FX model, between the original dual processor and the last dual core versions)..
It was slower than my brother's 1.8GHz core duo imac for most tasks. As strange as that sounds, the dual G4s always felt just as fast at the same basic stuff..
That said, I'd definitely get a mini.
Would a dual G4 run Tiger well? I had an iMac g4 and those things were very slow for me.
The thing is that while I am getting a new Mac mini I still prefer Tiger to Leopard and obviously, I won't be able to run that on the new mini..
I'm complicated.