Upgrades on the ICD Alley Cat

[Splat Attack]

I want to upgrade my alley cat, but I really dont know what to get. they have some pretty cool stuff on the manufactures sight(www.icdpaintball.com), but is any of it worth buiyng. I know I need a barrel, what kind?(J&J 2 piece any good?). Should I get a sight rail and a double trigger? Is an expantion chamber worth 50 bucks? Also, is a trigger job worth getting? What does polishing the internals do? I am new at the sport, Ive gone 10 times or so and love it. Just tell me what is worth getting.

Thanks