I think you would get the best improvement from switching to an SSD, the improvement in boot time and application loading or reading in your data files, is quite staggering. So I would do that first.
Then check on your memory usage. Use Task manager and performance monitor to do this. From a running system, press ctrl-alt-delete and click on task manager at the bottom of the list. Click on the Performance tab. This will bring up an overall performance view, CPU, Memory, Disk. That should give you a general idea whether the mempry is stresses. Click on the Performance Monitor button and this will bring up the more detailed performance data.
Again look at the memory but don't worry too much if it looks a bit higher than you expected - Windows uses a least recently used algorithm to free pages when the memory is stressed. If it sits consistently high, then extra RAM would help in two ways. More memory means you can hold more programs and data in memory and not have to keep going out to the disk for them and secondly, if you have the standard configuration on your machine, you have a 4 GB SODIMM and an empty slot. By putting a matching SODIMM into that empty slot, you will get dual memory paths which will speed up the memory access too.
If your memory usage is consistently low, then extra RAM won't buy you very much.