Well, when you come back from hibernate, the same thing that was there when you went into hibernation is there when you come back. If you wanted to clear the open programs, just shutdown instead of hibernating. Then when you boot, the memory usage would be less. I must admit, though, I hate shutting down. I just put mine to sleep.
But the hibernation itself won't increase memory usage. And I doubt your memory is "leaking" out. You might try running a memory test, just to make sure you haven't lost some of the chips to failure. I don't have a specific program to try, so unless somebody else has a recommendation, you'll just have to Google to find one.
If WoW uses 1GB and the other programs use 2-3GB, 3 + 1 = 4. And if the system (OS and other background stuff) uses a little under 1GB, there's where all your memory is going (4 + 1 = 5). That could explain it....
What is the memory usage after a normal boot? (before opening any programs)