I just upgraded from 2GB RAM to 4GB, thinking this was all I needed to do. But now it's telling me that Mountain Lion won't run on my machine. So am I stuck in the pre-iCloud world?
You may upgrade it to Lion if you wish, but frankly you're just as well off with Snow Leopard. But if upgrading is what you need to do:
Upgrading to Lion
If your computer does not meet the requirements to install Mountain Lion, it may still meet the requirements to install Lion.
You can purchase Lion by contacting Customer Service: Contacting Apple for support and service - this includes international calling numbers. The cost is $19.99 (as it was before) plus tax. It's a download. You will get an email containing a redemption code that you then use at the Mac App Store to download Lion. Save a copy of that installer to your Downloads folder because the installer deletes itself at the end of the installation.
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.
Are my applications compatible?