the better the sound card the better the sound quality. also you're source recording decive has to be good aswell. ie... good needles, mixer, records, etc. as the old saying goes, you can't polish shit.
soundforge does have a great deal of tools that can be used to clean up and 'master' as you put it your sound file. its not the top of the industry for mastering devices but its plenty adequate for what we need to do with a dj mix.