I'd say just buy a pre-built system. Custom made catbacks are ok if you're really on a budget, but the quality could be questionable. I'd spend the extra money on a prebuilt unit from a name brand, becasue they spend alot of money doing R&D on their stuff (you get what you pay for, you know?). Gibsons are nice, but aren't very loud compared to some. I personally prefer the lower volume that Gibson provides, because it isn't a pain in the ass on the highway but still sounds nice when you floor it. Other people like the really loud systems, but I think a Gibson dual exhaust combined with a Volant or K&N intake kit sounds really badass - you get the best combo of intake, engine and exhaust noise without one really overpowering the others.
Whatever you buy, opt for the stainless steel piping. It will last a LOT longer than the aluminized version. Trust me...I've made that mistake before.