|When you install ram modules that have different max mhz speeds, the mboard's bios will automatically run all the ram modules, max, at the speed of the slowest module. |
The mhz spec and the PC2-xxxx spec themselves are backwards compatible - a faster ram module will run fine at a slower speed.
However, there are also other things that determine whether the memory module is 100% compatible with installing it in a specific mboard model.
Memory modules that are not 100% compatible may each work fine in a mboard when installed by itself, but NOT work fine when one or more different memory module is installed.
What you're supposed to do is to go to a ram manufacturer's or ram distributor's web site and use a ram configurator or similar to look up which ram module part numbers they have listed (are compatible) by using your retail mboard model or brand name system model to search with.
If you already have ram, if it's brand name ram, usually if you go to that brand name's web site you can search using the module's part number, or search using the retail mboard model or brand name system model, and find out whether it's listed for your model. If it's generic ram, that may be difficult or impossible to determine.
If you DO have brand name ram, you should be able to determine whether only one module is not 100% compatible, or both are not compatible, by doing that. .
In any case, if the ram modules you have installed pass a memory diagnostics test, in your case when both ram modules are installed, then it should work fine with your mboard.
If you want to try a memory diagnostic utility that takes a lot less time to run a full pass than memtest86 does, this one is pretty good - Microsoft's
Windows Memory Diagnostic:
Windows Memory Diagnostic is limited to testing only the first 4 gigabytes (GB) of RAM.
It can be toggled (press T) to do a standard or a more comprehensive set of tests - use the default 6 test one first - if it passes one pass of that, use the latter one. A few of the tests in the latter set are intentionally slower.
If you don't have a floppy drive, see the Quick Start Information at that Microsoft link for how to make a bootable CD of the Windows Memory Diagnostic (you need Windiag.iso - you don't necessarily need to use the program they mention to add it to the CD).
The ram will probably NOT pass the tests when both modules are installed in your case.