What GPU are you planning on buying? That motherboard only supports one card, so unless it's a dual core monster, you don't need that much power. To answer the first part of your question, you can replace your power supply. It's one of the few things the OEM can't restrict you from doing. Just let us know what graphic card you like on Newegg or whatever, based on you budget, and we'll let you know how much power you need and what companies to buy from.
without a budget, but within reason:
Nvidia GTX 560 ti $245
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130634
CORSAIR Builder Series CX430 430W $45
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139017
Nvidia performs better in WoW than AMD/ATI card according to Toms:
http://www.tomshardware.com/reviews/world-of-warcraft-cataclysm-directx-11-performance,2793-22.html