MMORPG's generally have very very low requirements. The requirements of WoW, compared to something like Crysis... If Crysis was a 10/10, WoW would be a 0.5/10.
MMORPG's require many many people to be able to play them, and smoothly. So they rely very much on the gameplay, rather than graphics. If the graphic demands were too high, people would either stop playing due to choppy and laggy visuals, or just never attempt to play in the first place.
WoW could probably be played smoothly on an integrated 64mb (maybe, I really don't know. I've never played it). Building a machine to run Crysis on 1600x1200 high specs is an altogether different story.
With computers, everything changes very quickly. The technology is going forward at alarming rates. A lot of people don't notice this, as they're quite happy with their 5 year old computers... "It still does what I want it to." The thing is, there is a constant stream of new reviews, new chipsets, new GPU's to be reading about.
Upgrading a computer part by part over a year or something isn't a great thing to be doing, as you might buy a motherboard with DDR2 RAM, but then by the end of the year when you get around to buying the RAM, DDR3 might be very affordable, and offer a much better performance boost (It's currently very very expensive for very little performance gain). Saving your money is the way to go.
Also, what parts will you actually be needing? Will you need a new case, Hard drive, power supply, sound card, etc? Is it a full rebuild?