memory on an old Dell Latitude Laptop ???

I have a Dell Latitude CPi Laptop that is a 400MHz Pentium II. I have a 128Ram chip that is a Toshiba T8z93-80 16m x 64 SDRAM that came with it. I bought a "MemoryStock" chip 128MB SODIMM and I beleive it is a 66MHz chip. If I place the Toshiba in either slot "A" or "B" it will read it, If I place the MemoryStock chip in slot "A" it reads it as 128, in slot "B" the computer will not boot. (This is using the chips one at a time. If I place the MemoryStock chip in slot "A" (where it reads it and the Toshiba chip in slot "B" and boot. I get a message that it has a "Memory falure at 08000020 reading OEF00EE Expecting EF00EF" then the computer finishes booting but only shows 128MB total RAM.
I think it is the speed of the chips that is in conflict but am not sure.
Also Dell states That I can use 100MHz RAM but only in slot "B" (normak speed is 66 or 80MHZ) If I want 256 total I have to use a 256MB single chip. I beleive the 128 chips are 66MHz for the MEMORYStock and 80MHz for the Toshiba. I would like to use them but if I have to buy different RAM would I be better off buying a single 256 100MHz chip ?
I just use this on trips to stay in touch with email and light word processor type use so I am not realy sure if boosting the memory will change things all that much.
Any help or experience would be appreciated, Thanks,
Dennis