Upgrading iMac RAM

Okay, I'm looking to upgrade RAM on my Mid 2007 Model so I can upgrade to Mountain Lion legitimatly. The iMac runs 1GB (Yes Tragic) and I'm looking for a way of purchasing a 4GB stick to upgrade the RAM whilst leaving a slot free for a 2GB upgrade further down the line. Apple says it can only run 4GB but I know it can run more as a friend is doing it and has had no problems.

I searched around for 4GB RAM and found this
Amazon.com: Corsair 4 GB DDR3 Laptop Memory CMSO4GX3M1A1333C9: Electronics

Very cheap but it 1333MHz and I'm pretty sure my Imac cannot run more that 667MHz (correct me if I'm wrong) so I found this one

Corsair Mac Memory SODIMM 4Gb 667mhz Ram strip to fit White Macbook | eBay

My price range goes from £0-25 and I am looking for a 1x4GB RAM Stick that will work on my iMac. It is Mid 2007, 1GB RAM, 250GB HD


Any help would be appreciated

Your 2007 iMac can only handle 667 MHz memory but is reported to handle up to 6GB of memory with a 4 and 2 GB sticks..

You don't want to buy the faster memory because it might be work reliably with the slower memory speed and timing..

Performance Upgrades; FireWire USB SATA Storage; Memory, more at OWC is a great of figuring out what your iMac can support. A single 4GB stick is $95..