Firewire or USB DAC? What's the current state of the Union for $500 +/- ?

I know people have gone round and round on this, but...........
I'm looking to buy a DAC and have spent the past week trying to figure out what to buy! I need to find something in the under/around $500 range and have considered finding a Focusrite Saffire (still available online, but the model TAS reviewed has been discontinued) and using it with a Firewire Card from my laptop and with my Wadia 170i.  Also have heard pretty good things about PreSonus's firewire dacs.
The alternative is a Cambridge Audio Dac Magic just using the USB out on my PC and the SPDIF out on the Wadia. I've also been intrigued by the Music Fidelity V-Dac which Chris Martin gave pretty high marks to.
I've got a pair of Galt speakers, which are absolutely incredible (they are custom made locally for Wilson Audio in New Orleans - sells for about $1,200 a pair and blew away most others I listened to in the $2,000 to $4,000 range.  I have an OPPO DV-980H as my CD player (thanks for the recommendation TAS!) along with an NAD 326BEE integrated amp.
Just being able to use USB would certainly be simple, but I want the best sound possible for the buck.  i'm reasonably computer savy, so I can probably get through the firewire computer setup issues. 
Any suggestions on how to proceed?
Thanks in advance,
Robert