I will wholeheartedly second the idea that adding an SSD in place of the existing hard drive, if you don't already HAVE an SSD, is probably even more important that adding more memory, UNLESS you are running applications or doing heavy multitasking and you KNOW you are running out of memory.
In that case, obviously the faster storage isn't going to stop the lack of memory resources, so then memory is the obviously bigger factor. Otherwise, I'd do an SSD first, memory after that.