Hypothetical: Giving my 2011 MBP a "make over"

So I've had my MBP 13" early-2011 model for 2 years now and I was thinking...I should give it a new lease on life. (Probably not this year, maybe next year) and I was wondering all the ways to do that.

I read replacing the RAM from 4gb to 8 would let me run more apps at a time, but the other idea I had was to replace my HDD with an SSD.

Has anyone done something like this this? Have you experienced a real difference in boot up times or applications opening? What can you tell me about the benefits of an SSD over a new HDD?