Congrats on your purchase!
I have a MAC cat-back on my 1998 GT.
It is a 2-1/2" cat-back exhaust with 3-1/2" chrome tailpipes. It sounds mellow at idle, but has a nice deep sound when in the throttle...this is with the factory 6 catted h-pipe.
I recently installed an off-road x-pipe. Big difference in sound! At idle it sounds racy. Going thru the gears it has a definite growl that gets noticed..but I like it. You will also gain some horsepower by losing the cats.
I would not replace just the mufflers. I would recommend a complete cat-back system to ensure you have a true 2-1/2" size all the way back....unlike the factory crimped pipes.
I got my x-pipe for less than $150.00 at -
http://www.uprproducts.com
You can search around for your cat-back system.
The MAC system I have is around $325 but you could also try the Bassani (excellent system but pricey), Flowmaster, Spintech,
etc.
Good luck,
Ryan