Are you sure you want Pro Tools? it is hellishly expensive and some of the hardware units are known to have definite issues with reliability..I'd suggest you look around some music forums and read up on it before diving in.. Its a massive outlay to front and there are many alternatives to the path you seem set on..
Pro tools is a fine platform but its *industry standard* titel is not only under threat but beginning to slip bigtime these days.. If you are using a lot of midi Pro tools is defnitiely not the package i'd be looking at.. The midi implementation in Protools is rudimentary, and that is being polite about it...
The place to start is not so much which sofware you intedn to use but which hardware for recording with. The present protools range is ,at best, somewhat dated in its standards, at worst , by todays standards, stupidly expensive..That is part of the reason that producers are forever pushing the mixes and various busses through exotic outbaord gear, cos Protools own hardware really dosent sound as good as it should..
So, having said all that . How much atre you thinking of spending on a soundcard?? and lets take it from there...