to start your OS doesn't have anything to do with your bios they are 2 seperate issues. If its not being read as 256 in the bios then somewhere your having issues with eithe the ram or your bios or even the bios settings
you can try D/L the latest bios from
the QDI site and flashing your bios to see if that solves the problem or if you have any other 256mb sticks of ram you might swap it out and see if it works. Might even try to change the slot its in?? it doesn't make sense to me that it shows as 256 in win and not in the bios usually it would be the other way around