Using a laptop for production because it offers portability is like using a bicycle for your furniture removal company because it's easy to park.
If playing live consists of farting around on stage with a single VSTI over a backing track then you don't need some uber powerful laptop, but if you plan to produce the backing track on the same laptop then it will need to be reasonably powerful.
You can use a laptop for production but they are second rate compared to a tower. Apart from obviously getting the highest specifications in a laptop you can afford, look out for graphics that rob system memory and slow hard drives, you should actually have at least two hard drives (a system drive and one for streaming audio) external is an option but if you go with firewire keep in mind that you can't daisy chain an audio interface with it, also you might want to consider the idea of ditching the DVD drive to install a second hard dive instead, that way you can reduce the amount of shit hanging off your laptop.