Reddit mentions of Beyerdynamic DT-250-80OHM Lightweight Closed Dynamic Headphone for Broadcast and Recording Applications, 80 Ohms

Sony MDR 7506 or MDR V6 should be well under your price limit and fine for vocal work.

Sennheiser HD 280 Pro should be about $100. They clamp hard though, so not high on my list of choices, just from a comfort standpoint.

Beyerdynamic DT250 is right at your price limit, but another to consider.

Your first step should be to get off onboard sound. The two easiest routes here would either getting a dedicated sound card or a USB DAC/amp combo.

If you decide on upgrading your sound card, something like the Asus DG/DGX will do fine to start with; they'll clean up the sound nicely and will be good enough to drive an efficient set of headphones. Later when you upgrade your headphones, you can then upgrade to the ST/STX and get yourself a proper amp like the Matrix M-stage or 02. That would imo be the better option in the long run.

The USB DAC/amp combo like the Fiio E10 will be a better option if you plan on getting slightly harder to drive headphones from the get go, e.g. Senns, Beyer...

That leaves you with around $250-$300 for the headphones. For your uses(movies and rock), an open pair of headphones like the AD900 would be a great all rounder; great comfort and very efficient. Also have a look at the Beyer DT880, also a good choice within your budget. These would have a wide soundstage perfect for movies and be versatile enough for most music. An open set will generally sound more natural than a closed one.

If you really need closed, the Brainwavz HM5 would be a good choice and way under your budget, enough left over to get yourself a set of Grado SR80. Grados are known as the hardest rocking headphones around, with a fast energetic and upfront sound that goes great with rock/alternative/metal/anything fast paced.

As some others mentioned, the Beyer DT770 are a nice set but a bit too bassy for my liking. I much prefer the DT250-80, which have a great smooth midrange.

You can't go past the Beyerdynamic DT-250

Make sure you get the 80 Ohm version so you don't need an Amp to drive it, probably the best pair of closed headphones I have ever tried. The only thing I dislike about it is the curly cable which combined with the fact they don't fold makes them slightly less portable (look into the Beyerdynamic T50p for something more portable)

One of my favourite things about them is that they are incredibly comfortable even after quite a few hours use.

(This is from memory of an old audition)
Yes, I think it isolated well. Will you need a lot of isolation?

Here's some threads I found discussing DT-250's isolation:
http://www.head-fi.org/t/289245/isolation-of-beyerdynamic-dt-250-80
http://www.soundonsound.com/sos/1995_articles/dec95/beyerdt250.html
http://www.amazon.com/Beyerdynamic-DT-250-80OHM-Lightweight-Headphone-Applications/product-reviews/B0002V9EA4

My favourite headphone ever hands down are Beyerdynamic DT 250

https://www.amazon.co.uk/Beyerdynamic-DT250-Headset-80-OHM/dp/B0002V9EA4

So light and comfortable, replaceable parts, very neutral sounding. You can wear all day and not feel a thing. Love them and used them for about 5 years now.

Edit - I hate mechanical keyboards. No idea why people like that clunky shit

Personally use these 3, amazing for music and great quality - have used the first 2 for nearly 4 years, have had no issues so far!

• Open headphones, Audio Technica: ATH-AD900
  
  
• Closed headphones, Beyerdynamic: DT250
  
  
• In ears (best for bus/train/public transport rides, since it's easier to carry around): Westone UM Pro 10