If you can find an recent G4 iBook (14" 1.4 Ghz+ CPU) it makes a really good WoWtop. We used them last year for the laptop program here at the school and WoW ran very well and looked great due to the Radeon 9550 (I think) GPU. The series before it used a 9200. When we switched up to the newer MacBooks for the staff and engineering program, WoW took a very noticable hit due to the integrated Intel GPU (GMA 950). It looked crappier and played crappier, not much better then my 3 year old HP with an IGP 340m Radeon.
I play WoW on my MacBook from time to time, but it's a come down. Even WC3 runs fairly sluggishly. Could be that it doesn't have a universal binary though.
Now WoW on a MBP would look amazning. I have a Dell with an ATI x1400 and it looks great so I would imagine that running WoW (or anything) on an x1600 would look stellar.
All systems I've played on had a gig of ram as well. Any less hurts play greatly from what I've read.
That was my thought. Then again, I am having a hard time pulling the trigger knowing a new Macbook costs the same.
Maybe try to find a refurb or ebay deal? I will say the other advantages to the older iBooks are that they run considerably cooler (natch) and have a much better battery life (4-5 hours compared to about 2 on the MacBooks). The only problem is that they are pretty much now obsolete when it comes to the software. Probably won't be too long before support ends for teh PPC line. It would be a good deal if you could snag one for $400-500. Anything more I don't think I could reccomend.
Here's my model (Dell e1505) with pretty much the same or greater specs that I got back in May: $1,083 (Core 2 Duo instead of Core Duo and DVD burner)
PROCESSOR Intel® Core™ 2 Duo T7200 (2.00GHz, 4MB L2 Cache, 667MHz FSB) edit
OPERATING SYSTEM Genuine Windows® XP Media Center Edition 2005 edit
LCD PANEL 15.4 inch UltraSharp™ Wide Screen WXGA+ Display with TrueLife™ edit
MEMORY 1GB Shared Dual Channel DDR2 SDRAM at 533MHz, 2 Dimm edit
HARD DRIVE 80GB 5400rpm SATA Hard Drive edit
OPTICAL DRIVE 8X CD/DVD Burner (DVD+/-RW) with double-layer DVD+R write capability edit
VIDEO CARD 256MB ATI MOBILITY™ RADEON® X1400 HyperMemory™ edit
SOUND OPTIONS Integrated Audio edit
BATTERY OPTIONS 53 WHr 6-cell Lithium Ion Primary Battery edit
WIRELESS CARDS Dell Wireless 1390b/g (54Mbps) edit
So far it's been a rock-solid system. Big enough to be a good desktop replacement but small enough that it's not a bear to lug around. I just wish Apple could come down in price to match these specs or releasae OS X for the rest of us.