Which noise-canceling headphones should I buy?

[poincaraux]

I don't know a lot about audiophile-type stuff, but I'm going to buy a nice pair of headphones. I'll mostly be using them at work. We have a loud computer cluster near my desk, and I'd like some sort of noise-canceling headphones to get rid of the constant fan noise. Muting other conversations in the lab would also be nice, but I know that you can only expect so much from noise-canceling technology.

I browsed through the archives and found this thread, which had some good information. It seems like the best options are the Sennheiser PXC 250, Bose QuietComfort 2, and Sure E5c. There's one really annoying guy in my lab who always wants to talk about useless crap. The larger models (Sennheiser and Bose) have the advantage that they send a visible "I'm busy" signal.

So .. anybody have any advice for me?