None of these are multiplayer (except MW3). Just use your M50s if they satisfy you. Don't worry about what MLE said. 'Gaming headphones' usually means someone wants to hear footsteps around them to react faster in multiplayer.
Either way I'd say give the HD 598 a shot. It's a significant upgrade from the M50 for gaming and music alike.
Interested in buying a pair of headphones dedicated to PC gaming. I currently own a pair of M50s, which apparently are terrible for gaming according to http://www.head-fi.org/t/534479/mad-lust-envy-aka-shin-czs-guide-to-headphone-gaming-particularly-with-dolby-headphone I own a Tubemagic D1 DAC and use it as a headphone amp/DAC via the optical port of my desktop's motherboard. I could care less about price, going through a rut right now and could use a little pick me up so long as it's a worthwhile investment. I don't have a sound card nor a Dolby Headphone Amp/set up though I could pick one up if needed. As for games I play, Witcher 2, Dead Space 2, Mass Effect 2. I'm looking to buy MW3, Skyrim, and Mass Effect 3.