I have a pc clone with a 500 mhz chip, pentium 11, that had 128 meg
sdram at 100 speed.  The 128 was getting overwhelmed, so looked in the
pc, saw 4 memory slots (only one filled with 128 memory card), looked
in the motherboard book where it said each slot could take up to 256
memory for a total of about a gig, took out the 128 card and put in
two 256 cards for total of 512.  System only states I have 256 now,
not 512.  Book says no hardware or bios setup is required when adding
new memory.  Why doesn't it recognize the other 256?

can be several reasons for this 

a. one of the sticks is bad (try the sticks one at the time in a slot
and see if they come up with 256Mb each)

b. one of the slots is dirty , clean with compressed air)

c. the slot configuration is not 1234 but 1324 (so you need to put the
second strip in the  3rd slot)

d. the strips are not equal in type (eg. one has parity checking the other doesn't)

e. the strips are not seated properly 

These are just simple steps to check the most obvious things I can
look further if none help

please supply mother board manufacturer and type number 



e.

I think you probably read the manual wrong. This is common for older
boards to Max out at a specific memory count. In your case 256MB. The
motherboard simply can't address anything over 256MB. You might try
and see if the manufacture has any updated BIOS's for you motherboard
that might raise this limitation, but I doubt it.