I have a Dell XPS R400 with a Pentium II, 400 MHz processor running Windows 2000. (It originally had Windows 98). I currently have one socket filled with 128 MB of SDRAM (PC100, unbuffered, CL=2) . I would like to upgrade and add more memory.

Information from Dell says the computer supports 100 Mhz SDRAM 168 pin DIMM with a maximum size of 128 MB up to a total of 384 MB (3 sockets).

The motherboard is an Intel SE440BX-2. The documentation for it says it supports up to a 256 MB DIMM SDRAM for a total of 768 MB.

Why the discrepancy? Is it based on the OS? And how high can I go?

The old W98 did not function properly with 256MB or over.
W2000 does not need more than 512MB, but works fine with up to 1GB.

I would believe Intel rather than Dell.
Make sure you get PC100/CL2 SDRam, anything else does not work.
You can mix different MB-sizes memory-chips. So buy what you can afford.
Unless you do intensive graphics, even just 1 256MB-stick will do wonders.

DIMM order

Thanks for the response...

Is there any importance in installing the DIMMS in size order (i.e., highest MB DIMM in first socket, etc.)?

Have a look at Crucial, you can enter your system specifications & see which memory modules are suitable for it.

I'm going to be adding a 256 stick to a system that has a 128 stick in the first slot....Is it necessary to put the 256 stick into the first slot and move the 128 into the second?