The most straightforward choice is the O2/ODAC combo from JDS Labs or Mayflower Electronics. If you're handy with a soldering iron, you could make one yourself.
The O2/ODAC is the best value around, even today, if you want a benchmark class headphone amp and DAC. They run about $250-$280, but you'll never need anything else.
UPDATE: If you're trying to get a setup that's as flat as possible (I.E. works perfectly with everything you throw at it, and responds perfectly to EQ), the DT880s are a better pick. They're some of the best headphones around for both frequency and phase response.
I use a pretty cheap Muse DAC that I got on ebay for like 15 bucks, and it's pretty good. Better than the laptop audio chip for sure. I don't use an amp for my headphones but I use one for my bookshelf speakers, it's a Muse M21, which uses the TA2021 chip, also pretty cheap on ebay, ~50 bucks.
Take a look at Audioengine D1. I'm not sure if it will have enough power for 250ohms headphones. Another one is Fostex HP-A3. Good thing about these is that they are easily available from Amazon and could be returned if they don't work out.