Welcome to TF mate.
Both Seagate and WD are good, reliable brands really. I have always used seagate, however, many around here use WD and to a lesser degree Samsung [also good HDDs]. I presonally like seagate because they have never failed me and they come with a 5 year warranty from memory. I think WD come with a 3 year warrenty. Really, either brand its fine.
As for size, well it depends how much you need - thats up to you. But try and get a 7200rpm drive with a 32mb cache, some cheaper ones will have 8mb caches and arent really worth getting.