First of all RAM is measured with MB(and GB),and all 32 bit OS's recognize max 3.25 GB RAM.
So for full 4GB RAM support and more you need a 64 bit OS.
Also for WOW,you won't need 6GB or 9GB of RAM,4GB would be fine.
If you use 32-bit OS and application, you will have a memory limit of 2GiB per process; so WoW can use 2GiB maximum. The same is when you run WoW on a 64-bit OS, since WoW is still 32-bit. You need 64-bit app on 64-bit OS to let it use more than 2GiB.
However, any unused RAM will still be used as file cache. WoW accesses the disk while gaming, you may experience short 'hangs' as it accesses some data off the disk when something unprecitable happens, like another player with some rare gear runs into your field of view; then additional disk accesses are required. With lots of RAM, this would be kept to a minimum after some time playing, because everything is kept in RAM.
You would need 16GB (+2GB system) RAM to allow WoW to fully cache itself in RAM. But you can consider using an SSD too.
Lets not confuse the OP with a whack of techno babble , simple answer Kressark: 4gb of ram will do you fine , 6 is better aslong as they don't charge a premuim for it.Your main focus should be the CPU and Video card , I play WoW (I know guilty pleasure) and with my experience a fast clocked Dual core CPU and a strong Video card will be great.WoW is more demanding on the CPU with Raids and in Dalaran as there are alot of toons to render and data to collect from the server.
So if you could share the speccs of the system you are looking at ,then we can help you choose a system that has all you need for WoW . Because we would'nt want you to get a Sempron System with 6 gb of ram with onboard graphics lol.