The fact that you reboot and it fixes the problem is NOT that you're low on RAM. It is that you have a memory leak somewhere. I'd guess in your video drivers or possibly an application. To do a quickie check on applications, try this:

--> Play WoW until you start experiencing the problem.
--> Close WoW
--> CTRL+ALT+DEL and open Task Manager
--> When Task Manager opens, go to the View menu and select "Select Columns"
--> In the "Select Columns" window that opens up, put a check in the boxes next to "Handle Count," "Thread Count" and "GDI Objects."
--> Click OK in the "Select Columns" window.
--> In the main Task Manager window, check the box in the bottom left to enable "Show processes from all users"

Once you do this, size the window up a bit and see if you have any programs using up lots of handles, threads or GDI objects. Those are usually your offending processes. Once you identify which ones are being bad boys, you can take steps to upgrade them to more-system-friendly versions or disable them so that they don't impact your gaming. As an example, here's a couple of screenies from my work laptop to give you some feel for what default processes might look like:

http://l00py.net/misc/TM_Handles.jpg
http://l00py.net/misc/TM_Threads.jpg
http://l00py.net/misc/TM_GDI.jpg

Note that the mysqld-nt.exe process taking up 33k handles is a known behavior but is not normal for anything else. :P I've had two bug situations similar to what you describe, but affecting another program and/or the entire system. One showed >200k handles open for that one process, the other showed >3k GDI objects. Note that you should research what the "offending" process actually is before you kill it.

Hope this helps.