Couple questions about upgrading my 01 ext cab audio

I just ordered a single din alpine unit and I wanted to get some info about rms. the alpine is 18w and I read that using upgraded speakers with a higher rms could cause a problem? Forgive me if I'm totally off, I have no knowledge about audio. Also, if everything is fine and will perform normal will alpine spr- 68 6x8's fit in the oem holes? I'm am not looking to have a fancy sound system with amps and subs. I just want a basic upgrade from stock. Thank you.

 

It won't cause an issue if the RMS is not the same. The most that will happen is that at high volumes the sound may be lacking in different frequency ranges and/or muffled.
This site will tell you the sizes for every speaker in your vehicle: http://metraonline.com/
Speakers and radio will be a good "basic" upgrade from stock.
If you are not as satisfied with that as you wanted, consider a 4 channel amplifier. It will increase the output from the radio and push it to the new speakers. (Kind of like what's built-in to your home audio receiver, an amplifier) It will take the RCA output from the radio and more-or-less match the RMS of the new door speakers, if tuned properly.
Hope this helps! Keep us updated!

 

Got it installed a couple days ago. I have been wanting a remote start, so I went down to the audio place and they installed my radio for free since I got the remote starter. Sound is better than before, so I think I will leave the speakers alone.