That's speed: 45ns and 40ns (nanoseconds) You could use either speed in that motherboard.
PC800 never really caught on, so it is VERY EXPENSIVE today. You could purchase a COMPLETE NEW SYSTEM from Dell for the price of 512MB PC800 RAM.
I'd keep the computer ''as is'' and use it for less intensive work and purchase a new system for the price of the RAM upgrade.
My PC is not an off-the-shelf or major brand ordered item. It was assembled from chosen components, except for the memory and floppy disk drive. The motherboard is an Intel D850MVSE, with 2x256MB (512MB) PC800-45 non-ECC RDRAM. I don't know why the assembler chose non-ECC vs. ECC--I wasn't given the option.
I'm looking at increasing the memory to 1GB (D850MVSE can take up to 2GB in two banks of two slots each, and can use either ECC or non-ECC). If I do, I may end up with 2x512MB of either ECC or non-ECC, then try to sell the non-ECC I have.
I've had at least two online memory configurators scan my system, and they both limit their choices to PC800-45 memory. Neither offered PC800-40 as an alternative. I don't know if that's because I have PC800-45 installed, or because PC800-40 isn't supposed to be used with this motherboard. It seems that the "45" and "40" are latency specifications, and generally PC800-40 is more expensive.
How do PC800-45 and PC800-40 RDRAM (otherwise identical in speed and size) differ, and can I use PC800-40 in the Intel D850MVSE? I don't believe the OS, etc. is pertinent, but I'm using Windows 2000 Pro SP4, with all updates through a week ago.