Your iMac qualifies to run OS X 10.8 Mountain Lion, but with only 1 GB of RAM in your iMac, this is not sufficient RAM for running either Snow Leopard, Lion or Mountain Lion.
Your iMac can take a max. RAM amount of 6 GBs.
Every iteration of Mac OS X uses more and more CPU, GPU and RAM resources.
I would purchase install the full 6 GBs of RAM into your iMac, before upgrading to a newer OS.
Your system will run for the better with less strain and frustration.
Your Mac must meet Mountain Lion's requirements:
Your Mac must be one of the following models:
- iMac (Mid 2007 or newer)
- MacBook (Late 2008 Aluminum, or Early 2009 or newer)
- MacBook Pro (Mid/Late 2007 or newer)
- MacBook Air (Late 2008 or newer)
- Mac mini (Early 2009 or newer)
- Mac Pro (Early 2008 or newer)
- Xserve (Early 2009)
From here > Apple - Upgrade your Mac to OS X Mountain Lion.
If your Mac does not meet ML's requirements, you may be able to upgrade to Lion.
- Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
- 2GB of memory
- OSX v10.6.6 or later (V10.6.8 recommended)
- 7GB of available spare
From here > Apple - OS X Lion - Technical specifications
I have an iMac similar to yours but with 3GB RAM. I'm using Snow Leopard 10.6.8 and it's very good. Read about Lion here and decided to be very careful, avoiding being committed. Have several bootable clones on external drives, installed Lion on one of them, can start up on Lion any time but I don't think I want to use it. The point is the working Snow Leopard on the internal drive has not been interfered with in any way and I can just continue using it.
That's what I did. I bought a 27 inch iMac just before Lion was launched because I'm still running Rosetta. I was entitled to the free Lion upgrade so I made a bootable external hard drive and installed Lion on it - I have the best of both worlds. The only problem is that I know I will eventually have to switch over to software (I know that Quicken 2007 is no longer compatible with Lion) that will work with Lion. But I'm in no particular rush.
Intuit now has a $15 Quicken 2007 for Lion available:
In addition you can install Snow Leopard (with Rosetta) into Parallels 7 in Lion and continue to run your PowerPC applications concurrently with Lion:
[click on image to enlarge]
Full Snow Leopard installation instructions:
You still need more memory than you have to even run Snow Leopard.
So, you'll need to purchase both SL AND more memory.
SL needs, at least 4 GBs to run the OS acceptably.
If you want other applications to run smooth atop SL, you need the extra 2 GBs of RAM to do do.
So, my recommendation stands. If you upgrade from where you are now, install the max. 6 GBs of RAM memory.
Good Luck to you and enjoy your iMac!