Originally Posted by
Klypto
Vista innately is a huge RAM whore, and in fact Windows 7 uses much less on your system, but that being said the amount you need depends on what you have.
RAM is useful because the more you have, the more operations your processor can do at once by retrieving the necessary data (of course limited to how good your processor is, but albeit the general rule of thumb the more you have the better).
For someone with a x32 bit system (as opposed to x64 bit) you'd cap out at 4 gigabytes of RAM. That is, you could have more than that but the Operating System would only register 4 gigabytes total, regardless of how many "sticks" you had. if you have a x64 bit system, then you really have no cap, and can stick as much as you want in there.
Make sure that the RAM you get is compatible with your motherboard too, you need to get some that is the right Frequency and the right type (DDR2 and DDR3 are the most common tmk). In short, do a bit of research before you start messing with your computer =P
Once you figure that out, if you want WoW to run smoothly I'd get another stick of 2 gb and that'd put you at 3 gigs total. Should make playing smoother.