WoW is more cpu reliant than gpu which is why it probably ran bad. Your current rig might lag during large scale pvp but should be fine in small scale.
Most people like the ATI 4890 because it is a lot cheaper and can be overclocked to reach about the same performance as the 285 gtx.
if the 4890 isn't overclocked (you can buy one that is factory overclocked) or you don't plan on eventually getti…
ideally you could buy the processor, motherboard, etc now and use your old video card then buy a dx 11 card when they come out but if you are buying it pre built then a card like 4890 or better will run aion easy.
look at the 3d mark scores. (scroll down a bit for cpu's)
www.yougamers.com/hardware/stats/3dmark06/priceandperformance/
buying pieces yourself is a lot cheaper than using a custom site like iBUYPOWER.
Personally I would go with a AMD Phen…
http://www.xtremesystems.org/forums/showthread.php?t=218744
decent building guide.
btw some direct x 11 compatible video cards should be out in a couple months.