Speeding up a computer is about eliminating bottlenecks. It's all fun to talk about CPU clock, or whatever, but in the end, your computer needs to send something through several parts and the links between them in order to complete a task. It has to fetch the data from memory, move the data from the RAM to the CPU, execute the instruction, and write the result. If the data isn't already loaded into the RAM, it has to go send a request to the drive controller to go get it, then wait for it to come back with the data. If there's hardware acceleration involved, it has to send the GPU (or sound card, if that's the case) its work, and wait for it to finish.
What you want to do is look at the task you are trying to do, find out what it needs to pass through, and find the part that everybody else is waiting on. SSDs majorly speed up drive access, so loading things into RAM happens a lot faster. If you are waiting on that a lot (which most people are), then an SSD will help you.