I should have double checked that before posting. (Guess I was sleepier than I thought.)
going to reply again once I checked to see what to change in SaX2 and, as it turns out, the card setting is not alterable. I'd bet the same is true for Slackware and, probably, Ubuntu as well.
Ignore my previous post's statement about changing the driver to something generic like VGA. You'll still need to remove the MX440 for now just so you have a working system on which we can get the nVidia card properly recognized.
It looks like you'll need to force the change by moving or renaming the current X configuration file -- under /etc/X11 on my system (YMMV) -- "xorg.conf" so that X won't be trying to load the onboard chip's driver.
Now... the best way to do this, IMHO, is to configure your system to boot up into level "3" so that the X Windows system won't start automatically. This involves changing a line in /etc/inittab. As a Slackware user, I'm guessing you know about this. (If not, give a holler.) Trying to change the low-level graphics configuration while actually running in a graphical environment is tough. Once you've got the system booting to level "3" and you have a command line login like you'd normally see if you press Ctrl-Alt-F1 (or -F2, etc.), you should be able to shutdown, re-install the MX440, and boot back up into level "3".
A question: what is the name of the X Windows configuration program on Slack? It might be something like "xorg -configure" but it's been a while since I've ran Slackware. (I know it doesn't require creating and editing your own X config files and risk toasting your monitor any more.)
Once we can get the system configured to boot into level "3" using the MX440, you should be able to login as root and run the X configuration to create a new "xorg.conf" file. Test it by issuing
startx 2>&1 | tee startx.log
If it doesn't work, you'll capture the error messages in the log file and can examine them, post them here, etc. (Well, actually, there'll be a LOT of messages in the file but, if all goes well, none of them should be error messages.) Once X is working, you can reconfigure /etc/inittab to have the system boot directly into level "5".
Cheers... (And sorry about the mis-information in my first post.)