1) To elaborate on goobi's answer - you will not have two separate hard drives - you have a single drive and can create 2 partitions on that single drive. This will not really give you two back ups as both of them will be on the same physical drive, just on 2 separate partitions. If the drive fails - both of your back ups are gone.
2) A lot of folks are using the WD MyBook series of drives and I have been a long time user of WD drives, particularly their enterprise class drives (you'll know them by the 5 yr warranty they have). Agreed with goobi, go with at least FW 400 if you will be transferring a lot of large files on an ongoing basis. Also like the Seagate drives and the 5 yr warranty on their consumer version drives.
3) As to places for purchasing a drive, newegg is tuff to beat as far as the combination of price and customer service. OWC is another you may want to look at. I have used one of their external cases and the Seagate 705GB drive.
If you like to read and research your devices, you may like
this thread from my own research a few months ago. Pricing may have varied some by now, but for the most part is still OK.
And if you really want two back ups - this will require two drives, not a single drive with two partitions.