If you only have 2 memory slots on a P4 RDRAM system then you have the
32-bit wide 232-pin modules (in fact, it's almost certain that you have
an Asus P4T533 motherboard). In this case, if you have two 256 meg
modules, your only options are to add 512 meg modules, but you do have
the option of mixing one 512 meg module with one 256 meg module for 768
megs. (Even though it's RDRAM, these systems don't use modules in pairs,
so you can mix a 256 and a 512 meg module).
I can't argue that such an upgrade makes economic sense, however. While
the 256 meg RIMM 4200 modules are available at reasonable prices (you
can buy a pair of them, 512 megs, for about $100 on E-Bay), the 512 meg
modules are very expensive. Even when they show up on E-Bay (rarely,
but it does happen), they cost $150 to $200 each. At those prices, it
may be less expensive to replace the motherboard AND memory than to
upgrade such a system beyond 512 megabytes.
However, I have never seen a system as stable and reliable as a P4T533
(or P4T533-C), and with a 3.06 GHz CPU, they are very nearly as fast as
a system with an 800 MHz FSB (the FSB is slower, 800 MHz vs. 533 MHz,
but the memory is still faster, 1066 MHz vs. 800 MHz).
