(This is coming from someone who does audio engineering, so my tastes/preferences likely are quite different, even moreso than the average difference in sound tastes...)
I've owned/used nearly every consumer sound card available in the past 5+ years that wasn't pure junk, and a large number of "professional" ones as well. Almost all of them are an upgrade over onboard sound, a decent amount are a worthwhile upgrade relative to the cost, some of them are good, and a few are truly excellent/exceptional.
The best sound card out of everything I've owned and used, and which I use to this day, is the Creative Titanium HD. It's simply the most balanced in terms of gaming, music, film, and even audio production. It works as well with stereo speakers as it does with headphones, it has exceptional build quality (look "under the hood" and it's all better components than what I've seen in most $1200-$1300 and under receivers!), and while it doesn't offer true 7.1 channel surround sound, you can get semi-lossy 5.1 through the optical output.
It is, without a doubt, the best headphone and stereo card I've used, and yes, that includes the newer, more expensive ZxR (which I had loaned to me for a month by a member of a band I recently mastered an album for).
If all you are not planning on using surround sound speakers for your PC (and if you are, you shouldn't run them from the sound card's analog out anyway, as surround PC speakers are universally terrible; you need real speakers + receiver to get anything close to good imaging much less sound quality), but instead just 2.0/2.1 speakers and headphones, then stop searching, this is the card to get!
Second on the list would be the HT|Omega Claro Halo XT with the daughterboard, which is a phenomenal music/film and even production card, but it's not good for gaming IMHO, but the biggest blow is that it's PCI. I'd have trouble buying a PCI sound card these days...
A lot of people will say "internal cards are awful, you need external", but I can tell you that I've never once gotten a better MASTER using an outboard instead of internal (and you cannot hear what isn't present in the final master), and on two occasions I've had to completely redo my work because of intereference resulting from the USB bus....
Still, they are convenient, offer good features for the price, and the 02DACamp is a nice piece of kit.
If there's anything I can help you with regarding a specific card, just ask!
Oh, and YES, the 50's 100% benefit from better audio! I use Audio Technica products a lot, including the AD700's, AD700x's, A700x's, AD900/x's, and a few others for gaming and when I need a wide sound stage, in fact the only headphones I use more are my many Grado's (PS1000's and GS1000's for recording; RS1/RS1i's and SR225's/325's for pleasure)!
The ATH-M50 are an excellent set of cans, and if fed properly you will hear things you've never heard before even on your most listened to tracks, assuming these are your first decent set of cans 😄