I personally use a no brand headset because it captures sound (Or at least my voice) really well. You wont get stellar sound quality from a headset, so it doesn't make any sense to pay much for one. Microsoft Lifechat is a good set though, there the mic is worth the purchase.
Anyway I just wear the headset around my neck, propped up so that its in the right place then use whatever headphones I want. Sure might not win any fashion points but it works really well compared to getting a clip on mic which will have to be pumped up so high that everything else will be heard along with your voice.
If you cant find the ATH-M50 what about the Sennheiser HD25 SP? Those two headphones share similar characteristics, and price points.