a move to the trash is usually enough. If you're really OCD you can delete the directories with the application name in /Library/Application Support or in some (few) cases, /Users/<username>/Library/Application Support
Welcome to mac, welcome to sanity. Old programs you deleted won't affect your OS performance because there's no registry to get all clogged up and sluggish like in windows. Yes, even windows 7 has a registry. Yes, it will probably affect the performance of the computer eventually. Luckily, we don't have to deal with that!
registry = fail.