Here's the link to the tools page of the "Tech Support and Documentation" section.
http://www.cisco.com/kobayashi/support/tac/tools.shtml
Choose "Software Selector."
You can be prompted, or manually enter the hardware inventory, and it will list the compatible software versions for your hardware.
When you go to the "Download IOS" page, the description for each version will tell you how much flash and RAM are required for that version.
Depending on your company's policies, you may choose between an Early Deployment (ED) or a General Deployment (GD) flavor. GD software has generally been out for a while and is considered pretty much debugged and stable.
ED releases are a little riskier, but (after a few minor revs) are generally OK for most common utilizations.
It's usually a good idea to read the release notes for the release your intending to use (especially the "Caveats").
Good Luck
Scott
