Questions about an atmega32 serial-bus to RS232 with converter and different logic levels
Answer has been given by Eminem, see his comments below:

Eminem

I use the MAX232, and 4 0.1milifarad ceramic capacitors. It works fine for my uses.

Me

In the datasheet in figure 4. You have to put 8.5V on the 2 and 6 pins. Is this necessary? And do I need a logic level controller between the converter (attached to the µ-controller, 5V) and receiver (3.3V)?

Eminem

(I use 1microfarad and not 0.1mili as i mentioned above) I dont connect 8.5V to those pins. Just the capacitors, which are connected to the ground as well. My Vcc is connected directly to 5V without a capacitor there. Though i do have a decoupling capacitor between vcc and the common ground.

Me

This is very helpfull. Now my module uses 3.3V for VCC. Is this a problem for the RS232 communication voltage levels?

Eminem

MAX232 is used to convert TTL voltage levels into RS232 ones. TTL means 5v and 0v, and my RS232 are -9v and +9v. The MAX232 "flips" and amplifies voltage levels. But im not a big expert, so il suggest you will wait for others to say their opinions.