Q: Worth upgrading??

Not till you determine what's causing your slow down. How much total and free hard disk space do you have? What applications are you running when you notice the slow down? 2gb of ram is low....even for Snow Leopard, and especially for Lion, you might consider upgrading that before any OS upgrade.

You have plenty of ram and plenty of free disk space. You might try starting up from your Snow leopard install disc by inserting it and restarting while holding the C key. Select your language and at the the next screen go to the Utilities menu and select Disk Utility. Run Disk utility to repair your hard drive and if it reports that it fixed anything run it again until it reports that the disk is ok. While in Disk Utility check at the bottom of the window to see what it reports for SMART status, while not always reliable if it reports verified, if it says anything else it indicates a hard drive problem.

I would also create another user and try that account and see if your slow down disappears, could just be a corrupt user file somewhere.

Finally I would try a safe boot to see if it clears anything up http://support.apple.com/kb/HT1564

See:

Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X, 
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.

Although you have plenty of hard disk space and your iMac model meets system requirements to run Mavericks (and Mountain Lion), the amount of RAM that you have —2 Gb—meets only the minimum requirements. Consequently, if you have several files and apps open at the same time or you're using a graphics intensive program, for example, your Mac is no doubt gasping for more RAM. Your iMac would certainly benefit from increasing the amount of RAM to 4 Gb and no doubt would speed up your Mac as well as reduce or eliminate crashes owing to insufficient memory. Crucial.com or OthherWorldComputing.com are reliable suppliers of RAM, although you can purchase RAM from other sources.

Keep with Snow Leopard.   Lion was never the best received of OS upgrades and there are still many people wanting to down grade back to Snow Leopard.   (From Mavericks too, I see.)  More than that, check your apps.   The lion family will not accept PPC apps (based on Rosetta) so depending on what you have, you may have to replace them.   Expensive.

YOU are exactly right. I read the first post wrong as if he had 12gb of ram. SL should run ok in 2gb, not great but ok.

I'm very sorry. I misread your first post and thought you had 12gb instead of 2gb. I suggest you open and run Activity Monitor while you use your Mac for awhile...not just minutes. Pay attention to Pageouts, rule of thumb is if Pageouts are more than Pageins you need more ram.

Again I apologize, I think your slow down is due to not enough ram.

http://support.apple.com/kb/HT1342