A 600 budget with OS and Monitor is pretty tough. I am awaiting my parts right now that totaled out to 550 after rebates without a monitor, os, mouse or keyboard. I would check out craigslist, ebay, Tom's classifieds or deals through retailers/etailers on prebuilt systems. The problem is the monitor and OS kill a hundred bucks each. If you can find a monitor locally through craigslist (around me all the time) for about 40-50 bucks, that will save you some, and you could probably find an XP license pretty cheap, but Win 7 is the way to go. A simple kb/mouse combo is 20 bucks, but it wont be much, especially for WoW, which is very macro/function reliant.
If it helps, it depends on how you play WoW. I just started on my Toshiba A-205 (1.72 C2D, 2GB of DDR2, Intel GMA 945) running Win 7 and I get a consistent upper 20s to low 30's frame rate, which is fine. I have only been doing solo and small party stuff though, no group quests or raids, and I doubt the laptop could handle it.
Sorry for the long talk, basically, look for some steals to piece it together as you go.