My first advice is to invest insome well recorded music. Keep your ear out and dig around here at Head-Fi for some good recommendations.
What type of music do you listen to the most if any?
My tastes in music change as I learn more about what sounds more technically correct and as my gear becomes technically better.
I often stop by and look at the What are You Listening to Right Now Thread:
Here are a couple of tracks I suggest listening to with your current gear.
Klaus Peter-Hahn / JS Bach
Without getting into well recorded music it is going to be hard to see how good your current gear sounds.
DAC Option: To tell if a DAC will improve your listening experience first see if you can hear hiss, noise, bleep-bloops, or other noise in the background of your music or more importantly with your music off, but gear turned on and up to normal volumes. A DAC really improves the electronically noisy environment that is inside almost every PC, but to see if a DAC will help try to hear any noise. A good system will be dead quiet with the volume at full both on the player / PC and amp.
Amp Option: An amp does a few things most obviously give more output voltage. If your current headphone get loud enough there is a good chance an amp is not going to give you the wow factor you might be expecting. An amp can also better control the drivers and voice coils by delivering more current / voltage and having a lower output impedance. This typically improve the bass response of a headphone.
Headphone Option: The ATH-M50 is a great headphone for both its price and functionality, but it does have its limits. It is one of the better closed headphone because it sounds pretty neutral, delivers good low end thump while keeping everything else in check. It is not my favorite headphone for hi-fi listening because it sounds pretty lifeless at times and because it is closed the bass is not very realistic.
If you look into the headphone option the other options become important because many headphone require more voltage and current than a PC may deliver. There are many reasons to "upgrade", first and foremost should be sound quality improvement.
Take a look at HeadRoom's PC listening section:
Head over to InnerFidelity.com and dig around Tyll's Wall of Fame:
You may also want to look at the $200 and under amp shoot out here:
This amp measurement post is also interested:
Edited by NA Blur - 12/6/13 at 1:52pm