I have a Late 2009 iMac (4 RAM slots, 2 with 2GB each). I'd like to upgrade to 8 GB. Is it ok to just put one 4 GB module in one of the two remaining slots, leaving the last slot empty? I've heard this reduces performance slightly, but I'd like the last slot open in case I want to install more memory later. Would this really reduce the performance of the memory? I've been looking at purchasing the memory from Crucial (see below): http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=E16129C3A5CA7304 Any help greatly appreciated. Thanks.