Upgrading AEGs

I was wondering if someone could link me to another post or give me the basic breakdown of AEG internals. I was looking to get a new one, probably either of these two:

http://www.airsoftstation.com/srcg36elairi.html

http://www.airsoftstation.com/srcxm8grelai.html

But the main point is I want to learn more about the internals, whats good and how to upgrade before I do any buying at all.

http://www.mechbox.com/g36

Watch that, It is relatively identical to the SR36E

For the SRC G-36 that I posted, I've read a lot of reviews about the hop up being poor, even though it is a metal one. Should I be concerned about this? Generally how expensive are hop ups and how hard are they to remove and put in a new one?

the hop up is the easiest thing to trade out and switch, also they are fairly inexpensive.

first i would take it out, shoot it, adjust the hop-up so it will shoot more accurately etc, before you go and upgrade anything. IMO stock hop-up units in any replica get the job done.
If the hop-up is indeed poor. then just replace it. Only about 15-30 bucks.
and throw in some bucking and a tbb while you are at it.

these are by far the easiest performance effecting changes to make

Well said NN2H.

The src metal hop up for their SR36 is meh/okay. It gets the job done.