I uninstalled the SiS driver again.
Rebooted and when the new hardware wizard came up I selected the sisgr.inf driver at ..\Setup\WinXP_2K\ (in the location where I extracted the uvga3_373.zip driver).
Windows installed the driver and then I rebooted again.
Same initial situation again: 800x600 and 1024x768 only.
If I do the registry change that cp recommended it'll come back with an additional res: 1280x768, but when I try to change the display to it it'll come with the format not supported crap.
Maybe this Sylvania needs a special monitor driver. I've tried to find one on the web unsuccessfully.