The 1.8 GHz G5 iMac is fine for running WoW. Easiest and best improvement is a RAM upgrade. Lucky you, the G5 iMac uses standard DDR 3200 like a desktop, not the smaller SO-DIMM's that the Intel-based models use of today. You can go up to 2GB, and I completely suggest you max it out. RAM is sooo cheap these days, it's a no brainer. Just do it.

If you want to max the resolution to the natural LCD's size, plan on bringing down the effects a bit, if you haven't already. That machine should be able to handle medium settings with no AA plenty fine once you have the RAM (the lack of enough system RAM is hurting WoW more than you know). The 64MB on the 5200 Ultra is nothing fantastic anymore, but if you respect it enough to not destroy it with high settings, it will still do a good job for a while yet.

Besides, maxing the RAM makes OS X so much more responsive overall. OS X utilizes RAM so well, it has scalable performance increases the more you throw at it... right on up to 32GB and beyond.... so max it out for general performance improvements, as well as WoW performance improvements. It's a win-win IMO.