It will play just fine actually. Memory is the biggest and best thing you can do for WoW. My wifes G4 800mhz with 640MB of ram and a Radeon 9100 (I think) 32MB card runs it great in 1024x768, and my brand new iBook (G4 1.33Ghz / Radeon 9550 32MB / 768MB of Ram) Runs it very fast.
Go ahead and pick it up, it will be very playable, just get as much memory as you can!