Samsung RF511 - Replace HDD

I have a Samsung RF511 and for a while now randomly my HDD sounds like it slows down then clicks.
This doesn't affect anything I'm working on at the time, but I'm sure my HDD is saying replace me!

I have backed up all of my data, but if my HDD fails I won't have a working laptop.

So my question is:

- Do I need to replace my HDD...or is it something else?

- If I do replace, I would want to upgrade from 640GB to higher (1TB +). Does anyone know a HDD i could replace my "640GB SAMSUNG HM641JI"?

Thanks