OK, I did some research on this, and discovered how to permanently set the priority. You have to run WoW from a batch file. In Windows 7, open up Notepad, copy/paste this in, save as 'Hog.bat' somewhere nice, and then double-click 'Hog.bat'.
Code:
Start "WoW" /high "C:\Program Files (x86)\World of Warcraft\Wow.exe"
REM /realtime = highest priority
REM /high = high priority
REM /normal = normal priority
REM /belownormal = below normal priority
REM /low = lowest priority
This would work for all programs, not just WoW.
Replace the /high with whatever priority you want (try /high first though.)
Replace the pathname to WoW.exe, with the pathname to whatever program you want to run at a different priority.
Finally - remember to double-click the bat file, not the program itself!