Many in the forum are reporting huge improvements in framerates. Here's a step by step run down of the process:
cat > ~/Library/Preferences/com.blizzard.Internal.prefs
B. open Terminal.app (in Applications/Utilities) and it should provide you with a UNIX command prompt. What we're going to do is paste the text in above, which will ask the command line to create a file in your preferences directory named "com.blizzard.Internal.prefs", and will inject the two lines of text after it into that file. We're asking WoW to use "vertex buffer broker mode 4" which is the new code for PowerPC systems.
C. Paste the copied text into Terminal. (Command V).
D. Now hit Control-D (not Command-D). Control-D concludes the command we started in step C and closes the file.
E. All done ! Start WoW and see how it runs.
For more information check out the WoW Forum post below.