I have gone down the upgrade (psu, hdd etc) path with an early Thomson box and it was ok till something else went wrong.
It is only when you start adding up the individual upgrade costs that you then realise that you have spent half the price of a new box.
The benefit of the new Sky 1Tb (1.5Tb in total) box is that it has been designed to work as a unit where as upgrading an older box just seems to add possible conflicts of hardware and firmware (IMO).
I have stated on a previous post that my new 1Tb box is superb and that I consider it a worth while purchase.
I don't know how much you may be able to buy one from Sky for at the moment but give them a call and see - nothing ventured and all that.