New iMac i7 comes with 2x2GB. How big, if any, of a performance hit will result in installing another 3rd party 4GB kit (would give me a total of 8GB) or should I just suck it up and buy a 3rd party 8GB kit? I do not plan on selling my Apple RAM in case of warranty repair, so that option is out. I also will not be upgrading my RAM beyond 8GB in the future so I'm OK using 2GB sticks.