Like SVTguy said, most aftermarket parts don't do a heck of a lot for performance. Even my tune, which everybody raves about, didn't help my main gripe with the car, the terrible performance until the car's up to temp.
That said, my SRI sounds wicked, don't know that it added much in the way of HP. Exhaust, I wouldn't buy Magnaflow again, it's too raspy, I'm actually thinking of welding a glasspack inline to take it down. The clips of MBRP I've heard have been awesome, not that you'll notice much performance difference, but it sounds cool.
Rear mount and STS you WILL notice, and they take you to about $200. Personally, with a budget, I might focus on preventative maintenance items. Fuel/air filters, clean the MAF, check/replace plugs, drain the tranny and replace with RP fluid. Your car will love you, and if there was a problem there you were losing more power than you could probably gain with most aftermarket parts.