Starter Dac for Headphones...

i've done a bit of research on the net but since there looks to be a lot of market i'm a bit overwhelmed on the different opinions/needs,etc...
In your opinion, what are the best features that i need to look about dac for headphones? usb-powered is good enough? Digital/analog volume control makes a big difference?
I don't need a portable one, i mostly listen to my music on my computer and i found "portable" dac a bit ridiculous lolll. Anyway, which one would consider in my case if i have a budget at around 400, 500 max? Would you suggest me to try them out first, because i don't know if the type of music would change the choices i have... (i listen to trance music, house, trip-hop, downtempo, jazz, etc)
I thank you in advance for your advices it means a lot :)