You appear to have a fair amount of RAM but I'm sure your HD is holding you back. I would add in a second drive in this case a 2.5" SSD following this IFIXIT guide: Installing iMac Intel 27" EMC 2390 Dual HDD or SSD Drive. Then configuring it as your start up drive and have it hold your apps.
As long as you're not doing heavy CPU processing this should get you some zip. Sadly you have only a i3 based system so it is not the fastest CPU in this series. While you could replace it to an i7 to gain performance think about how you use your system. To decide if its worth spending that amount of effort to do that.
The older models are much more serviceable than the new 'Thin Series' which is why I would try sticking with this system given the cost of a new one.