If its going to be used to backup information, I wouldn't go with a single drive. Either 2 identical drives in RAID 1 or go for 3 to give you some better speed in a RAID 5.
In fact its probably cheaper to go for the 3x500GB drives to get you your 1TB then it is to go for 1x1TB.
1 x 1 TB=£144
3 x 500GB=£137
Just as an example, plus you get the benefit of having more secure data than if you had a single drive.
WHS will not work with 64-bit computers. Will not back them up, or use the software. You can still access all the files/printers etc. MS was making a connector for 64-bit, but then gave up claiming it will be in the next version. Which is a rip-off tbh. I still wouldnt swop it though.