Other than maxxing out the RAM to whatever your Mac can take it's unlikely you're going to get much of a speed bump.
Photoshop is obviously a RAM-hungry app and it also wants and needs to use hard disk space whilst working, so it would be worth checking to see just how much space you have left on the HD.
Uninstalling old apps you don't need/use will obviously release valuable disk space.
Check you don't have too many launch items at login. Also, do you have any anti-virus installed as that can often seem to cause slowdowns/problems with system performance.
There is not much you can do at this point.
Your model iMac maxes out at 3 GBs.
You simply haven't got enough physical RAM to run OS X and all of the apps you have running simultaneously.
OS X, by itself, can use up between 2-4 GBs of RAM. You only have 3 GBs of RAM. So, already that puts your iMac at a disadvantage as it doesn't leave very much RAM to run other applications.
Also, if you are starting to get low on hard drive space (25 GBs or less), this can cause slowdowns with your iMac,too!
It maybe time to consider getting an external hard drive to store your important data to locally. Do not just rely on Cloud services to store your important data. You should have some sort of secure, local storage for your data.
Then you can delete some of this data off of your internal drive, freeing up additional hard drive space.
more ram won't help if you're not running out... you can go to 4GB, according to owc
perhaps check activity monitor each time you get the problem - screenshoot the memory, disk & cpu sections if it's not clear what the cause is.
all those apps make it quite possibly a running out of memory issue, and once it starts paging out, things can get extremely slow.
oh thanks - the 1st owc link didn't mention that - but they do here
"Although limited to physically utilizing a maximum of 3GB, there is a performance benefit due to 128 Bit addressing when 4GB (2GB x 2 Matched set) is installed"