8GB is more than enough for Photoshop/Video editing. Make sure all your RAM modules are the same and use CPU Z software to check at what speeds the RAM is running. Many times, RAM underclocks itself due to the Intel chip. This can be easily fixed by going in the bios and selecting XMP profile from the overclock screen. Note that this isnt considered overclock if your RAM is rated for that bandwidth and forcing it to run at its bandwidth won't void warranty.