You are off to a good start.
1. no, one card will power 3 monitors. Look at the 57xx series. The problem comes in with boxing software. Most will not allow WoW to play on more than 1.
2. Depending on the rest of your needs, a 1 TB is close to best bang for buck these days. WD also has good drives
3. I would think about a Blue Ray, but any DvD will work. All the new WoW will come on DvDs.
4. Make sure to get a quality power supply, with room to power the graphics card you choose.
5. The few things you list are not under powered at all. a new Quad and 8 gigs will run it all.

It can be done for $1500, but plan for $2000, and grin when you don't spend it all.