For the past 15 years, every time I got a new computer I had high expectations. Going from a 386 to a Pentium 166 was a hell of a difference, but later, every time I changed, I was always a little sad some things were still not as fast as I expected. Almost 2 year ago, when I got my first ssd, it has been the first time in a long time where I have seen a HUGE difference. Everything loads faster, but also, the whole computer is much smoother and fast.
SSD's are worth it all the way. At least, get a 120g and install your os and main applications on it. Personally, I only have a 12g right now, and the only difference is that I must clean it on a more regular basis instead of waiting 2-3 years until I flush the things I could have flushed years before, not needing them anymore.