I know they Make a dual G4 CPU upgrade (G4 model 7448) that they claim that can come close to the performance (and even surpass) a C2D Mac, i would not think so in games cause the limit on a AGP bus vs PCIe.

i would not really know much on this cause i havent seen real world test's on a dual 1.8ghz 7448 G4 upgraded Mac.

but that upgrade is like $699 USD or higher but if you already have the system it could make for the systems life span to be longer. and run around the same performance if not faster than a CD or C2D mini,

i would love to see benchmarks of a dual 1.8 G4 7448 vs a C2D mini