Magnaflow/Carsound High-Flow Cat off Ebay 50-60 bucks
Mufflers depend on what you get, a flowmaster 40 series is about 60 bucks i think, and the 80 series(most people here have these) is a little more expensive.
the muffler you want would be the one for a camaro/firebird with the 3.8...its essentially the same setup, 2.5 inch inlet with dual outlets
as for HP gains...id say maybe 20 at most? really the exhaust manifolds are the most restrictive thing on these cars...headers baby!
as for sound
here is a link to a thread featuring my Magnaflow
and our old pal BonneMEMN has a lovely collection of Bonneville sound clips for your listening pleasure found here http://www.bonneville.fearthepearl.c....php?page=mods
and that reminds me, BonneMEMN if you want to use my sound clips feel free, i know you dont have any for an SE, or for a Series 2 Magnaflow, the files are still hosted on my site even if you just want to link to them or whatever
2000 Oldsmobile Intrigue 3.5L DOHC V6 15x,xxx miles "The Forgotten W-Body"
shaved badges, Eclipse HU, Pioneer TS-A1370R'*, Pioneer TS-A6971R'*, 20% tint, cleared corners, Silverstar Hi'*, 9005 Low'*
Fenderwell Intake, Hi-Flow Cat, U-bend delete, GMPP Handling Kit, MOOG endlinks, and 12-inch front rotors
1996 Bonneville SE RIP 157,751 miles, totaled 8/29/05