I have found that I can upgrade from 4Gb of DDR3 RAM upto 8 for £38. Is this worth the money? I am running Moutain Lion on a 21" iMac mid 2011. I do not plan on upgrading to Mavericks but will probably upgrade to the one after that. Is there any speed inprovent when running Mountain Lion with 8Gb over 4? What do you think?