I have a iMac OS X v10.4.11 and I would like to install the latest version to the operative system expanding first the memory to 4GB. What is the latest OS? How do I upgrade my iMac?

The current OS X version is Lion 10.7.4. Sometime this month Apple will release Mountain Lion 10.8.

What you need to do is:

Check that you meet the minimum requirements:

Lion System Requirements

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

Apple - OS X Mountain Lion - Read the technical specifications.

Purchase Snow Leopard DVD from Apple's Online Store - $29.00.

Update Snow Leopard to 10.6.8 using the Mac OS X 10.6.8 Update Combo v1.1 updater.

Access the App Store and sign in with your Apple ID.

Purchase and download either Lion - $30.00 or Mountatin Lion - $20.00.

Be sure you first backup your current system just in case. Before installing Snow Leopard erase your hard drive then install Snow Leopard from scratch.