As Igor has said, we need to know the specs of the machine.
Older computers often have size limits on hard-drives, for instance my old computer's motherboard would only recognise up to 137GB of space on each drive. Which with modern 300, 400 and even 500GB drives would mean a lot of waste capacity.
As for the different brands of hard disc, I would recommend a "Samsung SpinPoint" drive, or a "Seagate Barracuda" drive. They are well-performing drives, good value for money and are also very very quiet. If your computer can take a 300GB drive, these seem to offer the best GB/£ ratio at the moment.