wind2k doesnt see memory

Hi

I recently upgraded the memory in my pc. It's an (old) AMD Athlon
XP1500 with originally 256 MB memory. I removed the old memory and
inserted 3 other 256 memory chips. Both are PC2100. The bios
correctly sees the 780 Megs but windows (2000) still only sees 256. I
ran a memtest and this didnt show any problems. Anyone any ideas what
went wrong?

thanks,

Gert