Hard to say, depends how decent your current soundcard is, I guess. All soundcards do the function of a DAC, converting a digital signal to analogue (DAC=Digital to Analogue Converter), but they vary greatly in quality of chip used, grounding and shielding, noise, etc.
One problem you may have with a top notch set of 'phones is your sound card may struggle to drive them if they're higher impedence. A good protable DAC can make a huge difference in terms of separation and clarity, but the ATH-M50's can still be a touch muddy and bass-oriented.
I'd recommend aiming to get both, and maybe start with the DAC, but I'd probably stay away from the K701's. They will sound a lot more "open" than your M50's, but they lack richness in the mids and have a good deal less bass than the M50's, so they will sound very different to what you're used to with those (not sure if you're looking for a similar sound signature or not, though).
I'd recommend researching more or even posting a thread over at Head-Fi (head-fi.org). Ehile there are certainly some knowledgable audio gurus here on GH, there are a lot more people with experience over there.