Which upgrades to my iMac 2011 will improve performance the most? I have started a job that will require processing of large datasets using python and ArcMap (separately and in conjunction with ArcMap).  I have a 2011 27" iMac with the 3.1 Mhz i5 (3rd generation I believe) quad core processor, 20 GB of ram, and 1TB 5400 rpm hard drive.  I believe this machine could handle the work load adequately as is, but this is a part-time gig and I want to maximize my efficiency with respect to computing power and impress my employers.  I don't want to buy a new computer right now (maybe in two years or so), for two reasons: 1) funds are currently limited, and 2) I think with the right upgrades, in the right order, with a focus on increasing speed and then storage, I could use this machine for the next 2-3 years.

Where should I focus first?  I see the options as:


Replace the pair of 2 GB ram modules with a pair of 8 GB ram modules to max out my ram at 32 GB.  
Add in a secondary hard drive in the form of an SSD, to store applications and important inputs to different processing steps.
Replace the 1 TB hard drive with a faster and larger regular hard drive.  This limits the need to transfer to/from externals through the USB 2.0 ports.


Obviously all three options could be pursued, but is there some efficient order that gives me the greatest returns up front, allowing me to stop upgrading when the iMac seems solidly up to the task?
You are the best person to answer that. 

Use the Activity monitor and take a look at RAM and HD current usage. 

That will tell you what you need most. Usually the more RAM the better, that in turn also removes some load from the HD, but a fasted SSD could help in the initial step of loading the app in to the RAM.

So if you open and close lost of apps and save lots of stuff a faster SSD will help. If you initially open lots of apps, but then just use them, without need to constantly access the HD (SSD) then RAM size matters. See it this way: RAM is the supper fast (temporary) storage between the CPU and the Hard Drive. Without improvement in HD (SSD) the boot up and the shut down will not be faster, but during the normal usage with large RAM you will benefit.