I had a smiliar situation, with a Toshiba P1800-921. Could not get this to work properly with Mandrake. I tried 10.0, 10.1 and even the latest 2006 beta. No success. OK to be honest I got the vesa driver to work (If you could call that work, it was soo slow that it was not an alternative. Playing a game of patience got me all stressed :-))

Somehow I decided to change from Mandrake to Ubuntu, and it worked out of the box.... no messing around in xorg.conf needed at all.

Now the relevant part reads

Identifier " Trident Microsystems CyberBlade XPAi1"
Driver "trident"
BusID "PCI:1:0:0"

It seems like the drivers are different in
/lib/modules/2.6.8.1-24mdk/kernel/drivers/video/tridentfb.ko.gz
/lib/modules/2.6.10-5-386/kernel/drivers/video/tridentfb.ko

Dont know if this helps you, but I will stay with Ubuntu on my Toshiba.

Studying the Xorg.0.log I noted that the reason displays over 800x600 were found to be invalid is that they were out of the sync range detected by xorg. So, I set the sync rages copying what I had used in an older Debian installation, after which I get 1024x768 by default as follows:

Section "Monitor"
Identifier "Configured Monitor"

HorizSync 31.5-57.0

# HorizSync 30-64 # multisync
# HorizSync 31.5, 35.2 # multiple fixed sync frequencies
# HorizSync 15-25, 30-50 # multiple ranges of sync frequencies

# VertRefresh is in Hz unless units are specified.
# VertRefresh may be a comma separated list of discrete values, or a
# comma separated list of ranges of values.
# NOTE: THE VALUES HERE ARE EXAMPLES ONLY. REFER TO YOUR MONITOR'S
# USER MANUAL FOR THE CORRECT NUMBERS.

VertRefresh 50-100

EndSection

I'm running a new Xubuntu installation 8.10.