And just to avoid confusion, there is no version of Mac OS X called "Snow Lion". These are the versions:
Mac OS X 10.5 - Leopard
Mac OS X 10.6 - Snow Leopard
Mac OS X 10.7 - Lion
Mac OS X 10.8 - Mountain Lion
System requirements for Snow Leopard and Lion can be found here:
I am going to assume you have a pre-2009 era iMac with a vertical, side-access DVD slot.
Over last weekend, I installed Snow Leopard (10.6 - 10.6.8) on a friend's 2008 iMac that was running Leopard (10.5.8). With just Snow Leopard and no launched applications, the operating system took 700 MB of her original 1 GB of available RAM. A 2 GB minimum memory requirement prevented Lion (10.7.5) or Mountain Lion (10.8.2) installations. I recommended a 4 GB memory replacement (machine tops out at 6GB) as soon as possible.
The key take-away is that what Apple recommends is the minimum for the operating system functionality. It excludes the additional memory demands that normal work habits and concurrent application use require.
From the Apple icon menu, choose About This Mac. That panel will show you how much physical memory is installed in your iMac. That is the definitive answer.
The 2007 iMacs shipped with 320MB hard drives as standard. One variant came with a 500 GB drive. The 2008 models came with 250 GB through a 500 GB based version. Since Snow Leopard requires a full installation and not upgrade, you have enough hard drive. You will need to back up personal files and reinstall third-party applications before Snow Leopard is installed.
Once Snow Leopard is installed and all software updates applied, you can use the included AppStore to purchase Lion or Mountain Lion as upgrade paths from Snow Leopard. Again, your hardware must meet the respective requirements. Consider at minimum, 4 GB of memory.