i would recommend dealing with someone reasonably local and get a trial before you buy. Moreover make sure you balance your budget towards all the components. I do not believe DAC is worth more than half of your budget. DACs do not make that much of a difference in sound quality. Your transport and speakers/headphones make most impact.
Mac mini with a software of your choice (they all offer 15 day trials, make use of it). Solid-state HD, upgrade to 8 GB of memory and get an external hard drive (Recommend 5400 rpm Oyen FireWire 800 harddrive with a third party FireWire cable as the one that comes with Oyen sucks). For headphones Sennheiser headphones of your choice, buy the best you can. Spend the rest on a USB DAC, headphone amp, cables etc.
Svenjosh