There are plenty of gaming laptops below $1600. Alienware and Dell both have very affordable laptops (in the $1200-$1400 range) that will have the nVidia 6800 Go 256MB PCI Express. For $1600 you'll have lots of headrooms to select better parts such as bigger HD, more RAM, faster CPU, better screen, etc. Alienware m5700 and Dell Inspiron 9300 are what I would recommend based on best bang for the bucks. There are many other companies that sell gaming laptops such as Falcon, Voodoo, and Hypersonic, and they all have similarly powerful (or slightly more powerful) versions for much more $$$.
Other mobile video card alternatives are nVidia 6600 Go, ATI Mobility 9800, Mobility 9700, and X700 that you can also buy along w/ your laptop configurations, but none are as fast as the 6800 Go. But if all you really play is WoW, any of the above would be fine, but if you're thinking about running other games like FPS or future games, the 6800 Go fits your budget very nicely and at the same time able to run modern games w/o lowering your details and resolutions. 7800 Go GTX is coming out very soon, but they'll probably run you around $2000 for a laptop setup.
I personally have the Dell 9300 w/ the 6800 Go on it. It runs like a charm. I play FEAR, HL2, Guild Wars, & CoD2 on this and they all run very nice at 1024x768. The only thing you'll give up for a gaming laptop is the size of the laptop. They'll be 8, 9 lbs monsters w/ 17" screens. So if you want portability and average gaming, then go for 6600 Go, m9800, m9700, X700 instead.