How can I check how fast is my Macbook ?

I am not sure how to get a quantifiable result without installing a software application, and or just simply using a Stopwatch.

What I can say, is I was in the same boat prior to the release of Lion. My MBP was 3 years old and on the original install of it's OS, with an in place upgrade of the original OS. Prior to Lion coming out, I bought a new HDD, installed a fresh copy of Snow Leopard, and noticed a considerable improvement on the overall speed of the system.

It is quite possible that the new drive is quicker, however, I contribute much of the speed to a clean install, with only current (software I use) applications installed.

Sometimes starting from scratch is a nice way to get a new feeling system!!