This is for power leveling with a high level as your main.

I leveled a priest to 60 just doing quests. My main was a 60 hunter, Pre-Xpac, I found that it went very quick if I went into a zone and picked up all the quests I could, and using AotP completed the quests as quickly as possible. Once you finished a zone move to a next of a similar level. Id run instances only when I had every quest I could for it. this worked great until about level 50 at which point I found that grinding took over as a better way to get XP. Though I still quested because i find grinding to be so boring.

I believe my played time to 60 with my priest was 7 days. Which is not bad considering the class. His played time from 50 to 60 makes up the bulk of that time though because once the level difference comes within 10 you can no longer speed through quests as effectively. I believe his time to 50 was 2 or 3 days played time. Also i spent alot of time at 57/58ish running instances with groups in order to gear him up.

As for where I got my hardware, I just used a machine I had built in order to be a media center and converted it to play WoW. 2 years ago when i put it together it cost me about 400 dollars and it does an ok job of running WoW.

As a gamer I upgrade my main rig ever 2 or 3 years which means i have a good assortment of parts lying around which helps lower your costs. Your second rig doesn't need to be state of the art. You prolly want your main rig to be as good as you can afford, but you second machine can be just a hodgepodge of thrown together stuff that runs WoW 'well enough'.

If you don't have spare equipment lying around, look around the web for barebone systems that come with case Mobo and CPU and then get a smallish hard drive, a budget video card (that can handle the load), 512 ram (should be enough), a monitor (LCD all the way, there just as cheep as CRT now days and the space savings is a huge plus), plus an OEM copy of XP(you could look into getting linex for free and i have heard it runs wow pretty well, you'll need a program like WINE though) if your thrifty and do your research you can prolly get all this for 500ish bucks.

My main gaming rig is getting out dated, Athlon XP3000, Radeon 9700Pro, 1 gig DDR400, and I plan on upgrading that in the next year. I plan on taking that old hardware and building a dedicated Second box for WoW and retiring my media machine back into the living room where it was meant to be.