S/PDIF
S/PDIF is a digital format. On the Delta 66, there's an input and an output on the card (not the breakout box) for coaxial S/PDIF. This is a stereo signal that is carried on a single 75ohm cable with an RCA connector (S/PDIF can also be transmitted across a fiber optic cable, with a TOSlink connector, just like ADAT but stereo). You can use it to transfer digital audio from your computer to another outboard digital device (such as a digital effects processor) and back in, thus keeping everything in the digital domain. This will save your signal from an unnecessary conversion to analog and back to digital. All conversions will degrade the signal to various degrees, depending on the quality of the converters. The Delta's convertors are not bad, certainly for the price, and get decent reviews. Be aware that the convertors for the Delta 44 and 66 are on the card, not in the breakout box. Their being inside the computer makes them subject to picking up "noise", though this hasn't been a concern for me. If you're a purist, you could consider the Delta 1010, which houses the convertors within the breakout box.
You could also use the digital input to route in audio that's been converted by a high quality external converter (Apogee), or perhaps a preamp with a digital output.
I've read that the Aardvark cards have good sound, but heard their support and updates leaves much to be desired. Their convertors (on the card) are sheilded under a housing to minimize computer "noise". I've never actually heard Aark stuff though. Getting a separate preamp for your mics would be more flexible, and would allow you to get a higher quality preamp, perhaps with a characteristic sound.
Hope this helps you out some.
-marcus