Thanks for all the recommendations so far!
But I posted to see if anybody knows the command to mute ingame sounds when talking, I had it but forgot it >_<
After googling around a bit I found the following code (
source):
Code:
alias "+mute" "volume 0; +voicerecord"
alias "-mute" "-voicerecord; volume 1"
bind "MICROPHONE-BUTTON" +mute
So then I could use just a mike with my regular speakers? I don't want to cable a 5.1 headset each time I wanna play TF2 and after playing recable my speakers. And I don't want to miss the surround sound. The downside would be that talking too much would totally mute TF2.

:/
In addition we just had the electricity billing (

) so I think I'll start with just a mike instead of a full headset. I found a mike-only from Zalman for around 10 euros:
Normally you can clip it to your headset cable, but they say it also works if you clip it to your shirt. Has anyone tested such a thing? I don't want it to be cumbersomely. Didn't found much other mike-only-to-wear-at-your-head-thingies.
So... I'm right that most of you pals use a full headset? Hmm... I think I'll go to a local store first and take a closer look on the stuff.