Use all $200 and grab yourself ram and a SSD. SSD will have amazing impacts on general system responsiveness as well as severely cut down load times and program open/close times.
The suggestion of Ram is a good one. However, lets be a bit more/less specific on it:
1) How many ram chips are inside the computer right now. Does it have open slots? If so, try to find a model # on the ram thats in there and buy another of the exact same chip. Otherwise you will want to replace both. That corsair memory Pyranans suggested is expensive, go with an 8gb kit (2x 4gb sticks) of Gskill ddr 1600 (or even 1333). You can get these on newegg for $50.
2) SSD: Grab yourself a Vertex 4 120g for $99, never look back again
---------- Post added 2012-11-10 at 02:29 PM ----------
Also, for WoW: your processor is actually a weakness. I am also an AMD user, Intel is superior for blizzard games at all price points. $200 is not sufficent for a new CPU/Motherboard though. However if you get another $350~ together, grab yourself an ivy-bridge i5 and motherboard.