Seeking DAC advice

Hey All,

I'm looking to do an upgrade. My current system:

Windows 7 PC, WAVs and FLACs
E-MU 0404 USB 2.0 External
NAD C 316BEE Integrated Amp
Mint condition JBL 4425 Studio Monitors

I believe that the weakest link in the above is the 0404. Please feel free to correct that notion. Nonetheless, I was considering and upgrade to one of the following DACs:

Schiit Bifrost, possibly with Uber Analogue upgrade.
Cambridge Audio DacMagic Plus
NAD D1050

I listen to a variety of music, and value detail with as little distortion as possible.

My questions:

1. Most importantly, will one or more of the above be a significant upgrade over the 0404?
2. Is one of the above DACs the best upgrade option (in the $4-600 range) for my system?

Thanks in advance for your time!
Mike