Any idea what DAW you'll be using? I'm not sure ProTools would be an ideal choice as they tend to stick to the "cutting edge", i.e. it will probably require you to upgrade your computer to carry on using it after a short while. I'd take a look at REAPER in preference, as you can try it out for free indefinitely (until you feel guilty and hand over the extremely reasonable amount of dosh). There's another DAW out there now that's open source, too. It's called Ardour and I have yet to try it out, but it's free and looks promising.
Another thing that will be an important requirement will be an audio interface of some kind; you absolutely cannot rely on the native audio in a laptop for monitoring purposes and definitely not recording purposes. These start at £25 and rocket up into the £000's so it would probably be useful to know what you plan to do on the computer to see what fits budget/requirements.
Amount of RAM will probably depend on your DAW and whatever else you may be utilising (for example, some virtual instruments, like NI Kontakt, might max you out quite easily).