Need some advice with headphones and sound.

[cnico88]

Right now I own a pair of Philips HP170 (older model) which i use for my computer and a pair of Creative EP-630 noise isolation (very nice) for my ipod and other portable media. I am looking for something that will give me a crisper sound and a little more than the Philips HP-170 and something that also blocks a lot of outside noise while the music stays in my ears and other people next to me won't be able to hear it. So far I searched and I am looking at either the Creative HN-700 noise canceling and the Sennheiser HD280 PRO. I am not sure if with the noise cancelling you might be sacrificing some sound quality as there is a constant hiss or static, in that case, noise canceling is out. So I want you guys to help me out in finding a good pair around 50-65 USD (I know the HD280 PRO is more than that but I can get it somewhere for around that price). Also I mostly listen to Pandora (free) which is limited to 128kbps and I don't know if that will make a big difference. Most of my music however is ripped at 320kbps. The other thing is: Is my mobo not producing enough sound and should I get a decent sound card? If that's the case maybe I'll be looking at the Creative Sound Blaster SB0570 Audigy SE 7.1 which seems to get pretty good reviews. Right now I am not in a rush but would like to know these things before I upgrade. Last but not least: Is this upgrade worth it for me? Thanks guys, specs in the sig as always.