Unfortunately, any good mic you get is going to pick up the sound from your speakers, and even if you don't get feedback, all the ambient noise (gunfire, things blowing up, screams of the wounded, etc.) is going to make it almost impossible for other people online to understand what you're saying - trust me, I've tried it.
I pretty much always suggest Plantronics headsets for online gaming for just that reason - and you don't have to sacrifice much in the way of sound quality as compared to your 5.1 speakers if you get one of the better sets. The USB headphones especially offer some amazingly good sound quality.