I'm going to buy the SM81 or the BG4.1 microphone. With one of these microphones I'm planning to make some serious recording into my computer using my Soundcard. My aim is to edit recorded sound on the computer. Currently I'm using the SOUNDBLASTER LIVE! PLATINUM from Creative Labs as a soundcard. Am I going to loose soundquality when connecting my microphone to the mentioned soundcard using its large jack? Or do you know any existing problems with this soundcard when used for recording with mics?
We do not know what you mean by "large jack", but here is what you can expect with sound cards. Any normal sound card has a pretty high noise floor, meaning that the hiss level in the background of the audio signal is louder than it should be. This includes any Creative Labs sound card. There are a few speciality sound cards on the market aimed towards home studios have the audio quality needed for professional audio.
Will your Soundblaster be of decent audio quality? That is up to you. We all have different acceptance levels of what we think is decent audio quality and what we are willing to pay for it.
As for the PG81 vs SM81... The SM81 requires phantom power. We are not aware of any sound card that supplies phantom power, so the SM81 would not work for you. The PG81 can use either phantom power or an internal battery. So, if you are wanting to plug the microphone directly into your sound card, the PG81 is the only one of the two that will work.
We highly suggest bypassing the internal sound card and using an external USB sound card at: Connecting a Professional Microphone to a Computer