Well the big advantage of a nice pair of over ear studio headphones in the $100+ range is that they will last a long time compared to a PC headset. Potentially the rest of your life. The one thing that will break is the cable. Some high end headphones come with a replaceable cable. Unplug it and plug in a new one. Another advantage is you can have a shorter curly cable at your computer, then have a longer straight cable to reach the sofa, chair, or bed, to watch movies. You can also use higher quality cables to provide a superior sound. Opening them to solder is just a pain when the replaceable cables exist.
Many people are hacking headpones like Audio Technica and Grado to add a plug to make them removable. they do this because they hate the included cables. Seinheisser user a stupid 2.5mm threaded connector so people have to get a replacement from Seinheisser. At least AKG uses a standard mini XLR so you can get aftermarket cables of good quality, or a replacement direct form AKG at a good price.
As for using onboard sound for now, look for low impedance. Something closer to 16ohms will push easier than 600 ohms as a rule of thumb. But ultimately look at the sensitivity specs. That tells you how loud the headphones will be. You want high sensitivity for onboard audio.
To compare those Audio technicas with the AKG K240 MKII I have in front of me. The 500x's are 100DB/mv sensitivity and the AKG are 104. So the AKG are a bit better. I can tell you now the AKG work from onboard audio if cranked...but nothing special. A headphone amp definitely helps. Just to give you a spec to compare to when shopping.
I took a look at some brands and here's what I found:
AKG K240 MKII - already talked about
Audio Technica ATH AD-500X - Great sound, very comfy, nearly identical to the AKG minus the earpads and removable cable
Shure has some that are closed back but in this price are not as comfy as the first 2. Would have to move up to more expensive ones.
Grado make wicked nice headphones but they are dual sided non removable cables.
Klipsche is more into headsets and sport/in ear headphones these days.
Sony makes nice headphones, but I checked and they are mostly into Bluetooth, Wireless, and noise cancelling sets.
Pioneer isn't as big on headpones as they used to be.
Seinheisser has the 518 and 558 series. Good quality sound but it's the stupid custom 2.5mm threaded cable. You need the cable from Seinheiser. A real turn off.
Just from the Hi-Fi forums the audio technicas seem to be very popular. Also the AKGs are regarded as, if not close to, #1 in price/value/quality. Their mini XLR connector is nice, as is the the ease of getting nice ear cushions. You could even step it up a notch and go something better then the K240 for a really awesome headphone. AKG also makes a lot of open backs, while some companies do more closed back. AKG is owned by Harmon Kardon and it's their pro headphone division.
Are you sure you want open backs? Anyways Just some info to help make your decision easier.
As for the mic the modmic is nice. But an alternative would be the Samson C01U at the same price. It's definitely better quality then the modmic, but not as convenient. It'll pick up desk noise, etc... what is one alternative is to get the suspension for the Samson for $25, then a $25 microphone desk arm. Then it'll float in mid air like a broadcast studio. You can then position it in front of your face and reduce desk noise. And you can push it aside when you don't want the mic like with music and movies.
With the suspension and arm, a Samson will be around $100-125, the modmic is $50. I'm just giving you some alternatives, not saying the modmic is bad. The Samson is also a USB mic so it eliminates hiss, and being a standard removable USB mic it should last much longer than a mod mic. But you need the suspension and mic arm to make it as convenient as a modmic.