Starting with the peripherals, Steelseries is running a Cyber Monday sale which includes some WoW mice and mouse pads at up to 40% off. They also come with a free set of touchscreen controls for your tablet and free shipping. You'll also find some headsets and other accessories on sale at Steelseries. Once you make the move to a mechanical keyboard, there's no going back. Newegg has one on sale for $49.99 after rebate. They also have a 27" IPS monitor half price at $199.99.
PC hardware deals
I believe that the single greatest upgrade you can make for your PC's overall performance is to get a SSD (solid state drive). Even a small sized one will be enough to hold your Windows installation, some applications, and a few games including WoW. Games will load quicker, and your entire PC experience is smoother and more responsive. Amazon.com has a large list of SSDs on sale, as does Newegg.com. A good rule of thumb for SSDs is that anything 60 cents per GB or below is a pretty good deal, but you will obviously pay quite a bit more for performance brands.