Slate.com posts on how to get computer audio on the cheap using, in order of increasing price, the Griffin iMic ($39), the M-Audio Transit ($100) and finally the Headroom Total BitHead ($269). Only the M-Audio can be described as truly worthy of a high end system due to using the Toslink optical to connect to a stand alone DAC, though the Griffin does an admirable job and the BitHead is considered an excellent choice for headphones.

What Slate didn't mention are true USB giants like the Red Wine Audio USB Select or other options from Empirical Audio. Prices for true high end start around $500 but give you whatever outputs you want. Red Wine will integrate USB into any of their products while Empirical advertises adding USB to any stand alone DAC (that has enough room inside - Benchmark DAC1 owners are SOL).

USB Audio [Slate.com]
Red Wine Audio
Empirical Audio