Typically just replacing the Y pipe doesn't get rid of the cats, only if you get an off road system. If you're looking into a Y pipe that will bolt up to the stock locations, most of those are made to be emissions legal and keep the cat in it's origional position. It will make it louder because it will flow better, but it probably won't make that much of a difference. When I did my headers and Y pipe my car really didn't get much louder in regular driving situations, it's a lot louder when you really get on it, but as long as you can keep out of the petal I woudln't worry about the sound, even with headers. You'll maximize the usage of your new Y pipe if you get headers as well, once again, that's why it's typically a package deal sort of thing, and that way you only have to get under the car once. My opinion, save up and do it all at once so you don't have to go back and do it again.

Mt. Sterner, obviously the amount of gain that you'll get can vary depending on how good of headers/Y pipe you get as far as flow goes. A complete exhaust system can probably get you a good 20 or 30 HP though, especially on an LS1.