From my experience Seagate are very good as they are reliable and have a 5year warranty... saying this the one i have had was quite noisy.
Maxtor also make good drives they are affordable and very good, can't fault them......
The one other brand i would suggest to have a look at are samsung Spinpoint Hard drives they are in my opinion the best HD on the market, they are very VERY quiet!! all be it slighlty more expensive than competitors.
Well thos are all the brands i have tried so i cant comment on Western Digital etc...
Just remember you will need to get a SATA drive not the old school PATA (IDE), from what ive heard the installation of a new drive is very easy so go for it,
Shadow