Max memory for 21" 2007 iMac? Have a 2007 21" imac. friend installed 2, 4GB memory for me, Apple genius says that's what causing slow downs on my machine? when they tested it only had a 4GB &amp; a 2GB cards, so someone along the way did a switch on me. Apple says my machine can only handle 4GB total. i wonder if that's true? seems like it ran quite well for a long time, &amp; it used to corroborate that it had 8 GB memory, so something fishy has occurred. machine is running quirky &amp; would like to have max memory. Would appreciate some knowledgeable input...
Crucial, UK state that 4GB is the maximum amount of RAM that your iMac officially supports. 

If you put more in and it worked then this could be seen as a 'bug', and wouldn't be supported by Apple. Such abilities could also be revoked through software/firm was updates.