For a start I warn you the 20" HDX laptops are BIG. To play WoW maxxed these days withn a decent framerate at 1680x1050 or 1920x1200 you'd want something like an 8800 or 9800M mobile chip and a decent dual core CPU. Unfortunately with the new realtime shadowing etc, WoW is a fair chunk more demanding maxed than it used to be.
The 9600M will run WoW reasonably if you're moderate on the settings, but you have to remember laptop chips aren't as fast at the desktop equivalents, the 9800series M chips are a little slower than the 8800GT in general.
I'd also recommend a 7200RPM HDD if theres an option, as this is the bottleneck on most laptops, and it will help your loading times both ingame, and whilst loading programmes, but does consume a bit more battery power. BUT lets be honest, HDX series arent designed for mass mobility!
Also keep in mind the HDX series are basically desktop replacements, so they're typically a lot heavier and more a PITA to carry around than most laptops, and if mobility is key you might want to take at 15" gaming laptops like the one OcUK do. More expensive possibly, but fast and more mobile, not to mention £1200 will get you a very nice OcUK custom laptop that will rival most of the HDX series bar the ones with 8/9 series SLI cards.
