In a perfect world you'd want to replace every bit of pipe on the car to open it up.
Most folks go for a Cat-Back exhaust to start. The stock exhaust is 1 7/8" OD IIRC, not even 2"
2" ID would be a upgrade. I would NOT do 2.5" on a non-turbo engine. It'll be too loud, and you'll notice a loss of power. Mandrel bent piping is ideal but most shops don't have a bender, personally I buy mandrel bends online and then piece my systems together.
Once you do a cat-back, you can upgrade the downpipe and get a high-flow cat (or delete the cat if your locality doesn't have emissions) Some folks do this all at once, depends on your budget.