500 GB Seagate Barracuda® ATA100 7200rpm 8MB cache HDD oem (Manufacturer's 5yr Warranty)
This is dead cheap for a massive 500GB internal Hard Drive - only £135.10 including delivery! Imagine all the lovely files you could fit on there - your entire music collection, countless movies, endless PC games etc etc!!
Seagate make top-notch hard drives as well, and this one comes with an excellent 5 year manufacturer's warranty!