For something "quick and dirty" (and inexpensive - it's $40), try Miglia's Harmony Express:
http://www.miglia.com/products/audio...ess/index.html.
It's a sound card about the size of a flash drive that connects to one of your computer's USB ports.
I've got one and it's not bad at all, although some head-fi snobs will likely disparage it, but I think that it works quite nicely; it does improve the sound.
I just received my new Xenos X1HA-EPC amp
http://www.aptecpro.com/index.html and it's really extremely good! It doesn't have a USB DAC, something that many folks, here, say is needed when using an amp with a computer, but I've found that it provides full, rich, luscious sound running off of my Powerbook's headphone jack; I've read that the soundcards in Macs, and especially their laptops is better than most.
It's $220.....don't know if that's more than you want to spend, but I think that's it's primo, an amp to seriously consider.
I've been using my K-701s and DT-770 Pros(it's got 600ohm impedance!) with my Xenos/Powerbook setup for about 10 hours, now, and loving it!