Can't get A7A266 to work with 512 mb ddr ram

Hello there i am using the A7A266 motherboard for the athlon AMD 1.33 GHZ for awhile ive been using one stick of 256 ddr ram memory and it works fine but now for some reason when i went out to buy one more stick ( same type and company make) both chips when installed stalls windows when it tries to load. Ive check that both ram sticks do work alone and in differnt slots they work but when put together windows 98 SE stalls and wont load all the way. The bios POST boots fine and reads 512MB OF ram. Im wondering is there something in the bios i can set to make it work or is there a windows 98 setting that i have to do before installing the extra ram? please help! thanks! =)