Stay with stock until something breaks. Once your trigger group breaks, use CWI trigger group instead. The RATech steel bolt is around $100, the steel trigger group is around $130.
I suggest you check out these two links:
.:::RA-Tech Versus CWI WE M4/M16 Steel Trigger Kits- Which To Buy?:::.
http://www.airsoftcanada.com/showthread.php?t=132679
.::RA-Tech WE M4/M16 Steel Bolt Tutorial::.
http://www.airsoftcanada.com/showthread.php?t=130612
(RATech currently has the 3rd revision of the bolt released so some mods may not apply)