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.
I use the e7/e9 combo with my HD650's every day. I got it because I wanted a solution that I could use both at home and at work. I use the e9 at home (with the e7 plugged into it), then when I go to work I bring the HD650's and the e7 with me, and use that at work (you would need to convert the plug from 1/4" to 1/8").
I love the way they sound, the e9 definitely allows it to "open up" quite a bit more, you can tell it loves the power. e7 is no slouch though and when I'm working on other things (read: not listening music and focusing on the music solely), I can barely tell its not on the e9.
My ONLY complaint with the e7/e9 combo is that the e7's screen is TERRIBLE. It scratches SO easily. Mine looks like complete garbage now. Not a big deal though, the battery life on it is awesome, so much so that I can now grab the er4p -> er4s cable for my etymotics and listen to music amped on the go. The e7 is incredibly light, and I barely notice it (i usually carry it in my hand with my wallet, as my wallet has badges that I need to get into work regularly).
To be fair though, I haven't heard other amps such as the little dot. My old roommates Claro Halo XT (I think? the one with the built in headphone amp) + Sennheiser 595's sound wonderful.