Well, here I am again.
I was doubting myself for awhile whether I was going anywhere with this project or not.
Why such delay? Long story short: Left home -> Bought Mac without RS232 -> Sold stk500 -> Bought AVRISP MkII -> Had to set that thingie up to work with Mac OSX. Phew...
I started to port all those obsolete commands in the code to those now supported by avr-libc and got it to compile without errors. Hence I've decided that my program works
and since then I've been experimenting a little bit with the analog input circuits. I took a look at both the simple and the fancy one through a scope and found out that the fancy one seems to be the better. That is if you remove the 100 Hz LP-filter. But I've only had the opportunity to test the simple circuit together with the Mega8 yet. It made the LEDs blink but not in an satisfying way.
From what I saw on the scope (and on the led I hooked up to the output of the circuit via a voltage follower) I would guess that my problem is that the circuit trigger the AVR rather arbitrarily as soon as the guitar signal fades just a little in amplitude. Do you have any other suggestions?
Anyway, I think I'm going to try the fancy version as well and if that won't do the trick I'm thinking about using the built in analog comparator as some kind of "through zero"-detector.
This is my status so far. I'll report back later.