dollar for dollar... go for it!
It seems to me that adding an SSD results in the single greatest change to real world performance in your computer. Going from a 2.4 to a 2.66GHz processor in a MacBook Pro is around $300, and you might NEVER notice the difference. Getting to 8GB RAM can be pretty expensive too. What's questionable is how much of a real impact users will notice from these changes.
On the other hand, replacing your HDD with an SSD results in very noticeable improvements to performance. Frankly, I can't imagine buying a laptop today without an SSD.