K7600 will be fine with 640x480i.
I can't really follow what you are doing unless I know absolutely everything you have done from start to finish. That, and I'm not really the software support guy. I only piped in because you seemed to have a DVI-D hardware issue, but it was just a mis-reported video card model. I keep hoping Calamity will say something, but he is a busy guy and can't be expected to hold everyone's hands all the time.
What I'd do is start again from the beginning, and follow Calamity's guides for your video card to the letter. Do them in order, don't miss or skip any steps. Usually this sorts out people's problems.
The only way to work out if you are using the right drivers is to refer to those guides, which I believe are accurate. You don't need any "legacy" drivers afaik.
So that's what I'd do. Identify what guide and drivers to use, follow the guide. Some people seem to have trouble following the guides but everything is right there. Then if it still doesn't work, report exactly what steps you took from beginning to finish and where it seemed to go wrong. Pics help.
Otherwise, maybe you are somehow borking up the monitor presets somehow - in which case, just use the default standard Arcade_15 or generic_15 profiles.