Re: Upgrade RAM or SSD?
Launch Pad -> Other -> Activity Monitor.
Check the field labeled Memory Pressure on your Activity Monitor. Have a peak at it while you are running all of the programs you typically like to use. If you are getting an orange or red reading here, odds are you could use some more RAM.
Upgrading to an SSD will substantially improve your load times. You'll notice right away on initial boots of the machine but also when starting programs.
Yes, adding an SSD is going to involve replacing your old hard drive. I personally recommend that you back up your files and do a clean install of the OS though there are options such as cloning, which copy the data of your old drive to a new one, that you can use if you don't want to manually replace everything.
I'm personally of the opinion that you can't go wrong with more RAM. I use 16 GB in my Mini. Computers these days have so much RAM (4 GB starting usually, yours probably only has 2 GB) that typical users can run all of their programs on only 4 or 8 GB and might not notice any performance increases by going beyond that.
RAM + SSD upgrade combo is popular because it will speed everything up. Load times will be faster and more programs will be able to run at the same time before you hit any slow down. It isn't going to allow the CPU to work any faster when you are waiting on it to finish some work but even fairly old CPUs like the Core 2 Duo are so fast that you won't notice the CPU working to do basic tasks like web browsing, word processing, etc. If your graphic design work involves any rendering that will still take about as long as it did before. If you have a large photo library on internal storage, the SSD will be great for getting all those photos loaded up quickly.