Yes, you can mix. Usually it works without a problem, but sometimes you have to adjust the settings to the slowest common denominator. Ideal is identical modules, since you don't have to worry about tweaking their settings in BIOS.

In your case, those Corsair pairs are about as close to identical as you can get, so they should drop in without a problem.

Please note that going from 4GB to 8GB has almost no measurable performance improvement for most users. Unless your system is already consuming > 3 GB of active RAM, going beyond 4GB only adds a percentage point or two of performance to your system.

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