Open will not really isolate at all and depending on the headphone it could leak sound.
Closed generally does not leak sound and does a better job at trying to block noise too (but not perfectly).
I don't know what speakers you have, but the headphone jack on computer speakers is always pretty crappy. Does the audigy2 have a jack you can use as another line out? If so, use it.
The Audigy2 is not great either but I don't think you want to replace that right now.
When you say you could go higher if you wanted to, how much higher? I see you like rock, in that case I highly suggest the Alessandro MS-1:
http://www.alessandro-products.com/headphones.html
However if you can not stretch to $99 then get the Grado SR-60:
http://www.jr.com/JRProductPage.proc..._Code=GRA+SR60
It's $69 with free shipping. I believe the shipping on the MS-1 is also free.
If you have mainly been listening to your music on computer speakers, ipod headphones or sony headphones then you will be in for a treat with either of these.
They are both open however, if that won't work for you let me know.