Upgrade your HDD with a SSD
The hard drive is a common bottleneck in computing these days. This is due to the fact, that processors have become faster at computing. The hard drive cannot keep up with the CPU. Because it's slow, the CPU is not stressed as much as it could be: bottleneck effect.
See these videos for a comparsion of a SDD to a HDD in a Macbook Pro.
If your amount of RAM is insufficient, you will see lots of page-outs. Data which should be written to the RAM will be written to the hard drive instead. Because the hard drive's input/output operations are much slower than the RAM's, this slows down your computer.
However, if your amount of RAM is sufficient already, you will not notice a benefit from a RAM upgrade.
You can check how much page-outs there are using the
Activity Monitor.app which is located in
/Applications/Utilities/. There are no official recommendations I know of as to what the ratio of page-ins to page-outs should be. Generally, you need to know that the more page-outs you have, the better your system could perform if you were to upgrade your RAM. I myself aim for a ratio of no more than 5 to 1 (20%, page-in vs page-out)
Activity Monitor → System Memory:
If you installed the upgrade 'over the top' this will be like a 'software factory reset'. Mostly you'll notice a better performance because no other software has been installed yet.
Still, a clean install helps in removing remains of uninstalled software but should be an ultimate measure for tackling problems that do not want to be solved otherwise.