I was a new user of Linux with a troublesome onboard video setup (SiS 740). At the time, no distro would run X, at least not without a lot of work. I tried different live cds until I found one -- an older version of PCLinuxOS -- that handled my card at 800x600.
You might try different live cds to find one that works well, boot from it, then look at the XF86config and similar files used by the cd to see what works so well. You can save the files onto a floppy or other storage medium and even print them. If you install the distribution and there is a difference that prevents you from running X, you can compare them and perhaps overwrite all or part of the files that don't work.
From reading the thread, Knoppix seems to work for you from the cd.
I have heard that Mepis (Debian/Knoppix-based) has good drivers for Nvidia, and apparently the recent PCLinuxOS has some too. It's also based on Mandrake, with a good choice of apps.
Another option: get another video card, something a bit older so that it is likely to be widely supported. Until I found a solution to my lack of an AGP slot, I ran Knoppix 3.3 using an old 2mb PCI card from ATI and switched back and forth in the BIOS, as needed. I shudder at the memory of crawling under my desk to switch monitor cables....
Perhaps you might not have to switch, if it worked well for both OSs.
Just my 2 cents. No guarantees. Don't give up on Linux. I had my journey through the darkness of un-X, but I'm doing fine now.