Tuning decisions? expert advise...?

Ok, I've got a 06 daytona charger with a 5.7L when I purchased it, it was all stock except for the exhaust they told me it had flowmasters. I'm looking on getting more HP and better shifting points for my transmission. And maybe later getting some headers and CAI. I really don't know much on tuning and adding HP. I'm not sure which one would be better, I heard Diablo sport is pretty good, and I've heard SCT is good also. I'm looking on getting something that I can customize without hurting my engine, I don't think I would add anymore than these accessories I mentioned also something not too expensive. I would go up to at least 500$. I would get into the forum pages but my phone isn't all that great..