Its a 1.8 vs 1.8, or 2.0 vs 1.8, I don't think the performance difference is huge CPU wise, where the PowerMac wins is in upgradeability, you can add up to 4 GB of ram, a second HD, PCI cards, etc, while the iMac can hold 1 HD, (that is hard to replace I've read here somewhere), and you cannot change graphics card or add pci cards....
If this is important for the future, which I think is because you use it for graphic design as I do, you want to keep your options open for the future, what upgrading is concerned.
Either is a fine machine I guess, thought I cannot compare to the iMac because I have not used one.
Good luck.