Q: What can I upgrade to?

El Capitan, and possibly Sierra.

(142917)

If this is your iMac...

http://www.everymac.com/systems/apple/imac/specs/imac-core-2-duo-3.06-21-inch-al uminum-late-2009-specs.html

then you should be able to upgrade directly to El Capitan, if you meet the system requirements.

Upgrade to OS X El Capitan - Apple Support

Currently you can upgrade to OS X 10.11.x El Capitan. This is available at no charge from the Mac App Store.

However if you using 10.6.8, you also may have some very old legacy apps. These apps may not be compatible with El Capitan.  You can research using https://roaringapps.com/

Also, 4GB of RAM is considered the bare minimum for El Capitan, if your computer can be upgraded beyond 4GB, please consider doing so. If you are not sure how much RAM your computer can accommodate please check with either Crucial.com or OWC (www.macsales.com).