No matter what program you use, if you don't have a good computer, the progam will not work "good" for you. You will hear constant feedback and the program won't be stable itself either. You should have over 1GHz speed and 512mb ram atleast to be able to play stuff.
Guitar FX isn't the best thing to record effects, but it is great if you want to play live. If you want recording done then I recommend ReValver.