I'm currently running the Waves GTR3 on my home rig, which is Pro Tools 9 based. I've got a Mac Book Pro laptop that has a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM that I run everything on and have a Digi 002R for Pro Tools. I haven't had problems running GTR3, but on a lesser system I can definitely see it happening since it does take up some processing power. Make sure that you have a pretty stable system before purchasing GTR3.
Waves GTR3 is some of the most complete amp modeling software available. While in general I'm not the biggest fan of amp modeling as I like to get my guitar sounds using real amps, GTR3 can really help you extend on your natural guitar sounds or make some from scratch. I haven't used it with the optional Studio Guitar Interface, but with it you'd have a pretty good all in one set up. Having said this, the Studio Guitar Interface isn't going to be any better than most audio interfaces, so don't sweat needing all of it together. GTR3 software on it's own is quite powerful and definitely recommended for anyone in the market for high quality amp modeling software.