I was able to get my GeForce4 MX 440 working in Slack 9.1, after overcoming several problems, including the ascii garbage thing.
Then I repartitioned my hard drive and reinstalled everything.
Now it doesn't work. I get the ascii garbage again. And I don't remember how I got it working the last time. I did:
Driver "nv" --> Driver "nvidia"
in XF86Config like last time. I installed Slack from the same CD, got my kernel source from the same tarball as last time, and ran the same NVIDIA-blah.run from last time.
So my answer to your post from May is, yes, nVIDIA cards can work with slack. My question is, how?