OK, the problem is a blank screen then.
After you've modprobe'ed nvidia, and run startx (and you get your blank screen), then try ctrl-alt-backspace to see if you can kill X. If that doesn't help, maybe ctrl-alt-F2 will (swich to virtual console 2).
You need to get to a working shell, BEFORE you reboot. Because when you reboot, X starts up again (since you told Mandrake to do that... not always a good idea, like now), and your log from the blank screen run gets overwritten.
If none of that helps, you can edit /etc/inittab and change the id:5:initdefault: line to read id:3:initdefault: instead. Then reboot, and you'll be looking at a textmode login screen. Do the modprobe and startx, then when it fails, reboot but do NOT startx until you've copied the /var/log/XFree86.0.log file to somewhere else (to later attach here).
You can revert to a graphical login later, by changing the 3 back to a 5, but I'd be wary of that. If something else ever gets misconfigured with X, you'll have to do the same thing. Personally, I find it's not hard to startx on every login, but of course, you may not agree with me. Whatever.
And modprobe'ing nvidia then rebooting will accomplish exactly nothing. When you modprobe (or insmod, it's the same thing) a module, you're loading it from disk into memory. When you reboot, you're clearing all memory, so the module effectively gets unloaded.