Make sure that your horizontal & vertical sync information for that monitor is correctly setup in the XF86Config file. (That info should be listed in the monitors manual)
I run a 19" Philips monitor and the screen alignment for both Linux (RH8.0) and Win98SE are the same. (At home)
The Nvidia driver loaded will also help with this.
(The default nv driver will NOT align correctly)
As the 'nvidia' driver talks to the video card better hence gets the correct information from the monitor.
(Same goes for the Sony Multiscan 400PS I use here at the office. The screen alignment is the same for RH8.0 and Win2K)