Currently Being ModeratedDec 29, 2012 3:56 AM (in response to haggis7)
You cannot download Snow Leopard. It need to be installed from a purchased DVD available from an online Apple store. Here is a link.
Once upgraded, you can update to 10.6.8.
4GB of RAM will be sufficient. That is what I have running on my Snow Leopard and it works well.
Currently Being ModeratedDec 29, 2012 4:46 AM (in response to haggis7)
Good sources of Mac compatible RAM are OWC and Crucial. Your iMac can support up to 8 GB RAM with the following specifications: 204-pin PC3-8500 (1066 MHz) DDR3 SO-DIMM.