Questions about Creative Sound BlasterX G5

I wouldn't buy it at all, it has terrible driver software. Better to get one from Fiio.

Yes, the DAC improves the audio of the microphone. (Noise reduction and voice enhancement). However, this is only really noticeable with a bad microphone, if you have a good microphone it is hardly noticeable. (Your voice can also be changed, e.g. robot voice, male to female etc.).

Beyerdynamic Custom Studio (Over-Ear, Black) What about these? Are they also good for gaming? Because I heard that you can connect a microphone here or should I get the Modmic?

No, the Creative Sound BlasterX G5 replaces a sound card. Either you have one internally or, as in this case, externally. In my opinion, a combination makes no sense at all.