Cheap DAC for music server and cheap CDP?

I understand this question has been asked numerous times before in this forum, but I'd like some aditional input on the following two questions:

1.) Is there any DAC in the sub $500 that would considerably improve the S/PDIF stream out of my Digidesign mBox?

2.) At this price point, is there going to be any major sonic difference made to a cheap DVD-A/SACD transport?

Thanks in advance...