Yeah, it’s not recommend to mix different clock rates. Depending on your motherboard and which slots you use it might work, but you might experience issues.
Modules rated at different speeds can be run in dual-channel mode, although the motherboard will then run all memory modules at the speed of the slowest module. Some motherboards, however, have compatibility issues with certain brands or models of memory when attempting to use them in dual-channel mode. For this reason, it is generally advised to use identical pairs of memory modules, which is why most memory manufacturers now sell “kits” of matched-pair DIMMs. Several motherboard manufacturers only support configurations where a “matched pair” of modules are used. A matching pair needs to match in:
- Capacity (e.g. 1024 MB). Certain Intel chipsets support different capacity chips in what they call Flex Mode: the capacity that can be matched is run in dual-channel, while the remainder runs in single-channel.
- Speed (e.g. PC5300). If speed is not the same, the lower speed of the two modules will be used. Likewise, the higher latency of the two modules will be used.
- Same CAS Latency (CL) or Column Address Strobe.
- Number of chips and sides (e.g. two sides with four chips on each side).
- Matching size of rows and columns.