You don't need an overclocked i5 for WoW. i3s, i5s and i7s didn't even exist when WoW first game out, and the engine hasn't changed since. The graphical improvements are child's play compared to games that actually require and i3 or better.
His CPU is more than capable of handling WoW at Ultra, it's his GPU that's lacking. You can run WoW at Ultra with a good GPU on a Core 2 Duo at 2GHZ+. I do it with a GeForce 560 GTX @ 1980x1200 no problem.
First poster, to answer your question, buy a GeForce 660GTX, it will more than suit your needs.
http://www.newegg.ca/Product/Product...82E16814125443
^ Never goes beyond 59 C at full load in Firefall/WoW/Bad Company 2. Firefall is a game that looks 5x better than WoW, and I run it at max settings with 40+ FPS in huge battle scenes. The last time I tried WoW 2 months ago (uninstalled now) I was sitting in the middle of Valley of the Four Winds at Ultra with like 120+ FPS, and every quality setting turned at max from the Nvidia Control Panel.
Never buy a graphics card for its looks, buy it according to reviews. I personally own this card and it's whisper quiet and utterly badass.
The price doesn't show but it's 230$ + 7.49$ shipping.
Your monitor is 1600 x 900 max resolution that's nothing for today's cards.
For Nvidia cards the second number indicates the performance factor. Ex: 640, 650, 660, 670, 680, 690. The numbers ending with 40 and 50 are low end to entry level cards. The numbers ending with 60 and 70 are more performance oriented and a very good value/$ ratio. The number ending with 80 is for enthusiasts who want to run multiple monitors and such with insane settings and good FPS, and the number ending with 90 is for people with a lot of money.
The RADEON/ATI/AMD numbers have a similar pattern but it's fucking confusing. It's like 7790 is better than 7860 or some shit I can't remember but don't buy ATI their drivers are shit atm.