The decision to upgrade or update is entirely up to you!
I'd like to use the new features but afraid it will have adverse effects on speed, functionality, etc
Speed - Max out your RAM if you have not already done so. Download Mactracker, look up your model Mac, select the RAM tab. This free utility will tell you the maximum amt of RAM your model Mac can handle.
Functionality - Prior to doing any upgrades or updating you should make sure your iMac is in good shape.
Disconnect all devices except for the keyboard & mouse from the computer then do the following:
Boot up from your Snow Leopard Install DVD (or the system DVD that originally came w/your comp) while holding down the "c" key.
Select the language you wish to use, and then choose "Disk Utility" from the menu bar.
Your HD should appear in a panel on the left hand side of the window which opens. Select it and then click on the "repair disk" button in the right hand section of the window.
Once this process has been completed restart your computer and repair permissions directly from Disk Utility.
If Disk Utility is unable to complete the repair, you will need to use a stronger 3rd party utility like DiskWarrior, Techtool PRO (not Deluxe) or Drive Genius.
As for the software currently on your comp, make sure they will be compatible w/Yosemite by going to the http://roaringapps.com/website. This site will let you know if your software is compatible and/or you need to update or upgrade your software.
Read this link in its entirely---> Apple - OS X Yosemite - How to Upgrade
