mechanical hard drives are extremely cheap these days.
make sure it fits (a lot of 2.5" dont, it has to be a specific size of 2.5").
make sure its 5400 rpm (faster ones produce more heat = problems for ps3 and early death)
ssd drives make next to zero perfomance upgrade
you should be able to easily get 10x what you have already for less than £50.
Thanks for the reply.
I have another 40gb drive, so was wondering how big game downloads tend to be?

Running out of days in may for this to happen.