I don't want to sound rude but if you are not sure about needing an "OS" you may consider having someone locally that is a friend and has the time and knowledge assist you before trying to buy, build and configure your own computer from ground up.
Simply mishandling a motherboard can easily damage it as seen in many motherboard reviews on Newegg where the person complains about 3 DOAs or other failures when really it is just a lack of knowledge on the user's part.
WOW will play on almost anything but for decent graphics and decent frame rates.
http://www.newegg.com/Store/SubCategory.aspx?SubCategory=48&name=Desktop-Graphics-Video-Cards
Will do a very nice job at $54 (minus a $20 rebate) for games like WOW.
If you are looking for a video card with a bit more power for some of the newer 1st person shooters see the $100 mentioned above in an earlier post.
Other considerations:
Windows XP is fine for WOW with 4 GB RAM (XP 32bit can only use about 3.5 GB MAX) but if you go with Windows 7 64 bit WOW runs very smoothly with 8 GB of RAM even while running other basic applications.
(little background)
I've run computer repair/service centers, custom build all my computers and play WOW almost everyday.
World of Warcraft really doesn't need the resources that some games demand but if you are able to get above the 30 fps with the graphics set at MAX it is awesome from beautiful landscapes, to water that almost looks real all while playing PVP with no lag.
Good Luck but don't rush in...take your time making your choice.