I would actually leave that CA sportline stock. It's almost not worth it to upgrade a sportline. Leave it stock for indoor games and get a higher end quality gun to build a decent outdoor weapon. If you are on a budget and you must upgrade this gun, then I would upgrade the hop-up as Amos said. I don't think a tightbore barrel is neccessary. Internally, that gun could use some new metal bushings, a bearing spring guide, and a more powerful spring.
Externally, anything you add to it would just be for accessories (sights, RIS, etc.) to your preference. If you wanted a full metal gun, it would have been better to buy one in the first place than to hope to upgrade the plastic body in the future.
Play some games and look at other people's guns. That's the easiest way to figure out exactly what kind of a look you want or if the accessories match your playing style.
If you want to play around with accessories or the look of the gun without spending money first, just goto pimp my gun:
http://pimpmygun.doctornoob.com/oldapp.html