If I have 4gb of ram and if upgrade to 8gb will I have 12 gb in total

According to the information I find on the web a PPC iMac does not support 8 GB cards.

You posted in the PPC iMac forum so I figure that is what you are running.

You state you are running Mountain Lion which is not capable of running on a PPC Mac.

Please state exactly what hardware and operating system you have?

Allan

You have four (4) RAM slots. Typically, iMacs of this generation that ship with 4G RAM have two, 2G modules, leaving the other two slots open. So, yes, if your add two 4G models, you will ahve 12GB RAM.

To take advantage of teh internal architectue, it's best to install in pairs.

Instructions here:

Installing or replacing memory in iMac (Late 2009) and later

I've asked the Hosts to move your post it the forum for Intel iMacs. Not your fault--the forum labeling around here is somewhere between dismal and abyssmal if you are not familiar with the more techy side of Masc.