If you plan only 5 wow's 4G should be suffiecent for the OS and the applications, WoW is like 1/2G each iirc.

If you know that you will be in WotLK then just put in the newer data files (what 5G?), if you going to play in BC then the first 7G of data files. Wouldnt be hard to know where you plan to play that day. Some hassel ya but ...