Needing advice on a good mic

if you're looking at the m50x for gaming you might want to look elsewhere (notably at the dt770 or dt990). while they do have good audio quality (i own the m50s) soundstaging is quite poor on them but is much better on the beyers. the dt770 is overall similar to the m50 but better in every single way. (but you may want a decent soundcard or amplifier for them)

as far as mounting a microphone to your headphones there are not many options. there is the modmic of course, the cheaper moovmic, various lavalier style mics like the zalman zm1 or jerry-rigging some other boom mic onto the headphones with zip ties and a 3.5mm female adapter cable.

overall the modmic is going to be your best on-mic solution. be aware, there are multiple versions - the unidirection and omnidirectional. an omni will pick up sound from all directions while uni will pick up more towards the one side. see http://www.learningaboutelectronics.com/images/unipolarresponse2.jpg for an example of its pickup pattern.

see http://0.tqn.com/d/np/home-recording/p111-001.jpg for an example of different pickup patterns available for microphones. the best to use for sound isolation from a background in your case would be cardioid as it (mostly) ignores sound from the rear. HOWEVER please do keep in mind that the greater the sensitivity of the microphone the more noise it may pick up as well so its a double edged sword when seeking quality microphones (which are more sensitive) but also seeking to reduce noise.

the at2020usb, at2020usbplus, blue yeti or cheaper atr2500usb all have cardioid pickup patterns and can be mounted on a nice boom and shock mount and are good choices if you want a desk mic. remember proper mic placement: the mic should be much closer to the source of audio (your mouth) than any other noise generating object in the room. it should also be isolated from your desk or any source of vibration (from fans, typing, etc). shock mounts help against this while pop filters prevent against wind from your mouth speaking. READ THIS FIRST: while they do have cardiot pickup, they are also much more sensitive than some other mics (good for quality bad for ambient noise) so you may get ambient noise from them if your ambient volume levels from fans or other equipment is loud. this depends however on your setup and placement of said objects in the room.

there are lavalier microphones available in cardioid style as well although i cannot give a list of good models you might want to buy. i can say that most are meant to clip on to clothing although you can clip them on to your cable. most will need a short adapter cable while some are mono and others are stereo microphones depending on your needs.

microphones with noise cancelling might also be of help.

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short version:

likely the easiest for you to use would be the modmic uni-directional as this has better noise cancelling features and directional sound pickup.

its less sensitive than the omni directional version (in this case that is better for you), has uni pickup (cardiod is better, but uni should be fine) and is very easy to use without investing in any fancy setups or tripping over/having microphones in the way.