Q: I have to buy RAM for my iMac 5,1 but don't know which one?

Go to Crucial.com and enter the details about your Mac there and it will tell you what to buy.

Allan

You have a Late 2006 iMac. It's a Core 2 Duo. It's 17" or 20", as either are the same model ID.

Check out OWC or DataMem for product.

No. That iMac is not capable of running Mountain Lion.

Allan

To install the maximum RAM you would need to remove the two DIMMs currently installed and replace them with two new ones.

Lion is the last version you can install on that machine:

Lion System Requirements

1. Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
2. 2GB of memory
3. OS X v10.6.6 or later (v10.6.8 recommended)
4. 7GB of available space
5. Some features require an Apple ID; terms apply.

Snow Leopard General requirements

1. Mac computer with an Intel processor
2. 1GB of memory
3. 5GB of available disk space
4. DVD drive for installation
5. Some features require a compatible Internet service provider; fees may apply.
6. Some features require Apple’s MobileMe service; fees and terms apply.

If you need to purchase Snow Leopard contact Customer Service: Contacting Apple for support and service. The price is $29.00 plus tax. You will receive physical media - DVD - by mail.

Third-party sources for Snow Leopard are:

Snow Leopard from Amazon.com

Snow Leopard from eBay

After you install Snow Leopard you will have to download and install the Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the App Store.

If you need to purchase Lion contact Customer Service: Contacting Apple for support and service. The cost is $19.99 (as it was before) plus tax.  It's a download.