Is there a list of DCM or NRA Highpower Competition legal modifications or specs for rifles?  

I got my AR-15 recently,and would like to compete, and there are just SO many goodies that you can add or change, but I want to keep mine legal for competition.

thanks

You can write to either of those organizations and for a modest fee they will send you a complete book of rules (which includes allowed modifications).

This explanation may be too simple; if so, I apologize.

I assume you are speaking of AR-based "service rifle" competition, as opposed to "match rifle" competition.  Broadly, a "service rifle" has to appear (emphasize appear) exactly like a US service rifle (there is one exception for Post-Ban rifles with no flash suppressors).  Several armorers - Accuracy Speaks, Fulton Armory, Compass Lake Engineering - have web sites, and that will help you understand modifications.  Also, the Bushmaster on-line catalog carries some kind of CMP / DCM weapon, so you can look to see what features it has.

My $.02, but the most common mod's (in order) probably are: trigger job; improved sights; steel tube around barrel; match barrel.

Again, broadly speaking, match rifle is much less restricted.  "Space guns" are not uncommon.

Read the FAQs here, then print out the pdf
version of the rule book
at this link. It includes a section describing
legal modifications for the AR-15.

www.odcmp.com/Services/Competitions/rules.htm