Anumber of people have tried to increase the size of the hard drive, but it cant be done, even if bigger it still only uses 40gig.
As a byproduct of this it has come to light that the power supply is marginal, and the machine will only take drives with a low power consumption. As long as the drive you wish to use is low power and this probably means a 5400rpm model it will work. I believe there are drives with better bearings available which are quieter in service.
However there can't be many machines out there which are out of guarantee yet, so I wouldn't recommend opening it yet, indeed there is a trick to opening the box without breaking the fascia