you will definitely notice a big improvement in speed
Define "big".
Normal stuff (surfing the net, MS Word, Excel, typical photo editing stuff) = no difference
Photoshop = almost no difference.
Encoding of some sort = a bit of difference, maybe 15%. Sounds great, but in real terms, expressing the time savings in terms of time rather than percentage, you probably won't save much time unless your files are huge and you do this often.
Work in 3D, or play video games = you'll definitely notice a difference.
I went from a 1.83 GHz Core Duo to a 2.0 GHz C2D, and I notice a bit of a difference, but it's small. The only reason I got the newer system is because my 1st gen MacBook was a lemon, and Apple replaced it for free. Otherwise, I wouldn't have replaced it.