That isn't entirely true...
If your machine will take a standard low density 128mb stick of ram it MUST take a true low density 16/18 chip 256mb stick of ram (this is due to the geometry of ram)
I have had low density 256mb sticks running in Intel TX systems on up.
However your machine will NOT accept 512mb as they are only available as high density.
If it does not take a 256mb low density strip it is due to a bios issue as the 810 chipset supports 256mb sticks properly.
If you needed 512mb of ram I would get a 256mb low density strip off pricewatch (make sure it is low density for intel) and see if it works, if so you could have 384mb of ram as you set.
Good Luck
Ryan