Question : Problem: What kind of rdram can I mix?

I have an old computer that I want to put back in order for my son. It has a MSI 850E mainboard with two 128mb kingston kvr800a 16-4/128 memory sticks. I wanted to upgrade it to 512mb. As far as I can tell, these may be PC800 40ns, but I'm not sure. I have 2 samsung MR18R0828AN1-CK8 modules (PC800 45ns)  I also have some PC700 45ns ,PC711 45ns and PC600 53ns. Will the machine work properly with 2 PC800 modules @ 40ns and 2 PC800 modules @ 45ns? Can I mix PC 800, 711, 700, and 600? What problems will occur?

Answer : Problem: What kind of rdram can I mix?

You can mix RIMMs of different speeds --> but cannot use any with a lower speed than the system is designed for.   In other words, if the system was designed for PC600 modules, you could use PC800 (they would just work at the PC600 speed);  but if it's designed for PC800, you cannot use PC600.

So ... in your case I suspect you must stay with PC800 modules (assuming the currently installed modules are the right speed).

If I understand your comments correctly, you already have two additional PC800 modules ==> so just install those and you should be fine.   Note:  You won't damage anything by installing them ... even if they don't work.   But don't discard the CRIMMs ==> they can be difficult to find and you will definitely need them if you ever want to remove any memory modules [as I suspect you know, all of the RIMM slots must be populated -- either with a memory module or a CRIMM].