I finally made it!!!!!!!!!!!
Now I got 800x600 resolution. I played around with timing settings and it worked. here are my X -probeonly command's and XF86Config's content (respective sections of them):
X -probeonly
(**) S3: Graphics device ID: "Primary Card"
(**) S3: Monitor ID: "Primary Monitor"
Warning: The directory "/usr/X11R6/lib/X11/fonts/local" does not exist.
Entry deleted from font path.
(**) FontPath set to "/usr/X11R6/lib/X11/fonts/misc:unscaled,/usr/X11R6/lib/X11/fonts/75dpi:unscaled,/usr/X11R6/lib/X11/fonts/100dpi:unscaled,/usr/X11R6/lib/X11/fonts/Type1,/usr/X11R6/lib/X11/fonts/Speedo,/usr/X11R6/lib/X11/fonts/misc,/usr/X11R6/lib/X11/fonts/75dpi,/usr/X11R6/lib/X11/fonts/100dpi"
(--) S3: PCI: Trio64V2 rev 16, Linear FB @ 0xe0000000
(--) S3: chipset: Trio64V2/DX rev. 16
(--) S3: chipset driver: newmmio
(--) S3: card type: PCI
(--) S3: videoram: 2048k
(--) S3: Ramdac type: s3_trio64v2
(--) S3: Ramdac speed: 170 MHz
(--) S3: Using Trio64V2 programmable clock (MCLK 50.114 MHz)
(--) S3: Maximum allowed dot-clock: 135.000 MHz
(**) S3: Mode "800x600": mode clock = 36.000
(**) S3: Mode "640x480": mode clock = 25.180
(**) S3: Using sparse 32 bpp. Color weight: 888
(--) S3: Virtual resolution set to 800x600
XF86Config
Section "Monitor"
Identifier "Primary Monitor"
VendorName "Unknown"
ModelName "Unknown"
HorizSync 31.5-36.1
VertRefresh 54-60
Modeline "800x600" 36 800 832 976 1016 600 604 606 634
Modeline "640x480" 25.18 640 664 760 800 480 491 493 525
EndSection
Section "Device"
Identifier "Primary Card"
VendorName "Unknown"
BoardName "S3 Trio64V2/DX (generic)"
EndSection
Section "Screen"
Driver "Accel"
Device "Primary Card"
Monitor "Primary Monitor"
DefaultColorDepth 32
SubSection "Display"
Depth 8
Modes "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 32
Modes "800x600" "640x480"
EndSubSection
EndSection
I've modified a bit your settings of 800x600 mode and it worked. It flicks a little though. I'm still not sure if it's the optimal setting for my monitor, but it works so far.
My next question is regarding the KDE settings (I think it should be the subject of another thread): I set up the desktop under root (all the kdelinks, icons and so on). I want to know where does KDE store all those settings and is it possible to copy them somehow in order to use those settings under my other ID on my machine. I have two IDs: root and other username.
I'd like to thank you again for valuable help