Check out the TVT250 kit, sold by, you guessed it: TVT Design. He's a member of our forum and attends a lot of east coast meets. Here's his site:
www.tvtdesign.com
Search a bit here in the forum and you'll see a lot of positive comments about the TVT250 kit.
Another excellent addition is a NeedsWings cold air intake. Rob (from NeedsWings) is a good guy and has been developing performance parts since back in the Neon days.
www.needswings.com
Another option is an ECU tune. There are a few vendors for these, LET (
www.letmotorsports.com) and InMotion (
www.inmotionusa.com) are probably the most popular. I don't know what kind of gains an NA Crossfire can expect from a tune.
Past that, there are not a lot of perfomance parts for the NA Crossfires. A lot of the aftermarket performance stuff is SRT-6 only and focused on the supercharger.
As for the transmission: that depends on which tranny you have. If yours is a manual transmission, I don't know. But if it is an automatic, I believe it is the same tranny as in the SRT-6. There's a few guys here in the forums running pretty hot cars (10.x and 11.x second quarter miles) with no mods to the transmission at all. It's a very stout tranny.
If you're looking to add serious power to the NA Crossfire, there is a user here "ChuckNorris" that developed a supercharger for the NA. But he halted that project and has since started a turbo project. I believe his plan is to make a turbo kit for NA owners to buy. But that is likely several months away, if it works.