Or this: http://www.diyaudio.com/forums/head...igh-performance-headphone-amplifier-pcbs.htmlHere's a site I like to open up every now and then, should help you with some suggestions. They definitely focus on portable products a little more, but there's still good information on there.
If you're in to DIY or open to the idea, check out the O2 headphone amp on diyaudio.
You can usually find the Aune (HiFiDIY) DAC/Amp on eBay for $150-160. I've heard they sound phenomenal for the price. You'd want to get the newest revision possible with whatever possible upgrades you can. Hooks up to your computer via USB.
You could also consider an internal sound card, if that's an option. I'd recommend one of the higher end Asus Xonar cards or something from Auzentech (like the Bravura).
There is also the Fiio E7/E9 combo that goes for $200 or so, I believe. That's very popular and apparently sounds fantastic.
If you're not looking to spend as much, I'd check out the Creative X-Fi HD USB external sound card. Really a great little device that will have no problem powering most headphones up to 300 ohms.
These are all $200 or less options. I'm not too familiar with higher-end options.