Well, downsized(sold) my system due to lack of space last year and while wanted to stay in hifi decided to keep dac which i`m 90% using to listen music through headphones.
Those computer speakers sound better through dac but big improvement was adding chord superscreen main cable to it especially in bass area, and while I would agree with you that dac is overkill if used only on computer speakers it still allows me some good quality through headphones so much that is probably gonna be upgraded very soon.
As for change of video card, is there supposed to be difference in quality even tho both dvi & hdmi are digital if I`m correct?