There's 7 things you want to look for in headphones:
1. Comfort.
2. dB range. Lower to higher is better.
3. Build design, and build quality. Overall appearance.
4. Sound quality, bass/treble levels.
5. Surround sound? Noise canceling? Around ear? On ear?
6. Replacement parts. Do they make them? Is it easy?
7. Cord length.
(there's a few other things, but that's all most people will care about... the hardcore ones are gonna care about distortion, leaking, and how loud it can go)
The HD555 are in Sennheiser's
Audiophile line, which has really good dB ranges, and are very comfortable. Sennheiser stuff is always high quality too, and easy to replace parts. So after 2 years or so if you need to replace your ear pads, you can just pay $20 to replace them instead of another who knows how much for a new pair of headphones.
The range on those is 15-28,000 Hz, which is lower and higher than the human ear can hear. So that means it can achieve any sound range effortlessly.
I'm getting the
HD 515's which have a 14-26,000 Hz range and are a fair bit cheaper.
Unfortunately, neither of those are good for gaming because in gaming you want surround sound, especially if you ever play FPS, so you know where people and/or things are. You can't really do that with stereo since it's just left or right.
So you need to get some headphones or a headset that has Dolby 7.1/5.1 Surround Sound, they make a few. But alternatively, you could also get a box that does that for you, though not quite as well. I'm getting the
Ear Force DSS because it has better bass quality than the Astro Mixamp and I won't need a mic, which the Ear Force DSS doesn't work with if it's not the same brand, but the Mixamp does. But, the DSS is stripped down and quite a bit cheaper. It'll do what I need it for.
Realistically, for video games ideally you'd want ones with surround sound and a mic. But, don't go with Logitech G35 no matter what. They're horrible. You'll have to look around if you decide to go down that route.
Also, as you look at higher quality headsets, you really need to look at what sound card you have on your computer. Because if you're just using on-board sound then you aren't going to be making much use out of the HD 555 over the HD 515 and you're probably wasting money.
Oh and I have a Bose AE2 and I'm not impressed with the sound quality. The ear cushions die fast too and they're $20 to replace.