Increasing RAM To 6 GB In A Mid-2007 iMac

I have a mid-2007 24-inch iMac with 4 GB of RAM and I recently upgraded to Mountain Lion in order to be able to utilize 3 & 4 TB hard drives. The problem is, the system seems to be sluggish as ****.

I've been told that while 4 GB is the most RAM this iMac is supposed to take, it's possible to increase it to 6 GB by simply replacing one of the 2 GB slabs with a comparable 4 GB slab.

I emailed Crucial and was told that "no - you can't do that - it might prevent your computer from booting", but that sounds suspiciously like a "company line".

Has anyone done this with this particular set-up? Is it necessary to replace the other 2 GB as well to assure compatibility? Has anyone had any problem booting back up afterwards?

I have a 2.4 Ghz Intel Core 2 Duo with 4 GB 667 Mhz DDR2 SDRAM.

Thanks for any and all input.