Sorry to burst people's bubbles about 10-15 gains through exhausts beeing done but
1999 Mitsubishi Eclipse RS - Power Pages - Import Tuner magazine
there are some more realistic numbers. A thing to keep in mind though, is that the more air you put into the engine, the more important your exhaust becomes. While there set up they only gained about 2-4hp with a doen exhaust, if you through in bigger TB/IM cams then your exhaust will give you more hp. Or if you turbo your exhaust could give you closer to 10-15 hp. Its all about getting air out as fast as possible, if you arent sucking much in, no real need for bigger exhaust.
If you plan on turboing I would get a 3-3 1/2inch. If you stay all motor i beleive that 2 1/2inch is the best set-up.
For sound, i reccomend going to youtube and searching eclipses and there exhaust and choose from there what you like best. Also, your intake will change the cound aswell as your catback.