memory timings
This is not necessarily for the original poster but good information for all.
If the timings of the memory are the same one MAY be fine. If not your motherboard MAY automatically configure to the lower settings. There is a lot more to it. I suggest doing a search on memory timings.
There are many instances where the system will not boot when using memory with different timings.
I would recommend to always use memory chips of the same make and model with the same timings. This will generally ensure that you don't have to mess around in the BIOS or actually lose speed. There are other factors from model to model that can cause trouble. DRAM frequency enters into the equation as well as command rate. Buying memory strictly on size to save pennies is absolutely insane and often results in money down the tubes. At best , it MAY run at a slower rate. Worst cases will not post, will not boot and possibly even fry motherboards and other components.
Matched sets are best. Using others can be a very slippery slope.
One should also check in your motherboard manual for compatible memory although this can often be outdated.
It is probably best to check with memory manufacturer's web sites for better answers regarding Memory. How do I know? I learned the hard way. Luckily I could sell the oddball memory for near the purchase price. Don't let it be you.