No A: drive if I upgrade memory to 512MB

I have just purchase a Dell Optiplex GX110 866Mhz Pentium III. For some strange reason when I upgrade the memory to 512MB using 2 256 DIMM modules Windows NT 4.0 doesn’t see the Floppy Drive. If I one single memory module or any other combination of memory modules as long as it doesn’t equal 512 MB I can still see the Floppy drive. Any Ideas?