Seagate has easily the best warranty. I would be inclined to go with an external case and put in a drive myself, but if you're not confident in your abilities, you may want to buy a ready-made one. (It's not hard to put them together!)
I'd recommend getting on NewEgg's mailing list. Just a few days ago, they had an internal Seagate 1tb drive for $80.
Still, I wonder consider your alternatives - like online backup. Mozy is $5 month (cheaper when you buy one or two years at once, and you can find discount codes online too - supposedly I can even give a 10% referral discount via my account but I don't see how right now

)... there are plusses and minusses to online vs an external drive, but IMHO unless you have a slow internet connection and are backing up massive quantities of data, online backup wins every time. Once you set it up, there's literally nothing to do. No worry of the drive going bad, no manually configuring backups (which can be a real misery sometimes)... it just sits in the background and takes care of things. (And it does support Macs.) Carbonite is another alternative though Mozy generally gets the nod as being the better product.
Remember that with hard drives, they can continue to appear to work for a while after they're beginning to fail, too, and you won't notice unless you actively test the drive. (Many failing hard drives used for your operating system are only noticed when a necessary system file gets lost and the computer won't boot anymore; with the nature of backups, you're unlikely to notice the drive failing until it completely gives up the ghost.)