Can I upgrade the RAM to 8 GB?

Hello,

I have early 2008 8,1 iMac with two 2+2GB modules. I would like to upgrade it to 4+2GB.

My questions are:

1) can I keep one of my old 2GB Kingston modules and add just one Crucial/OWC 4GB module to this configuration? Will it be stable? Or do I have to buy matched pair 4+2 made by one manufacturer (OWC, Crucial) to ensure the stability?

Is it right to insert 2GB module into the left slot and 4GB into the right slot? Or it does make no difference?

2) Of course I would prefer to upgrade to 8GB too, but let's say I buy matched Crucial pair 2x4GB (CT2KIT51264AC800) but the system doesn't recognize 8GB, is there any chance to run at least 6GB on the same configuration without further modules changing? Or will i have to throw 2nd 4GB module away and use another 2GB module instead?

Thank you very much for your help