lol, i think it would be best if you got 4x250gig harddrives, that way you can have 3 to depend on, BUT having 4 harddrives in one case can be troublesom because they might not be enough space, and even worse they will some what over heat because they so close together and there won't be enough air going through to take away that evil heat
Drives that size normally have heat problems under load, so expect failures in the first generation of products. If you do get it, make sure you have a solid backup solution or don't care about your data.
Internal drive: not yet. wait a while and pay dearly.
External: Get a chassis with JBOD/RAID and put a couple of drives in. some chassis' are quite easy to use with various options for connecting (UPNP, etc) via RJ45/USB
I don't recommend drives bigger than 300GB - their failure rate is frightening and they're more expensive per GB than the smaller ones.
I've heard this from people who buy hundreds of large capacity hard disks and Google released a paper about it last week. http://labs.google.com/papers/disk_failures.pdf
The sweet spot with regards to price per GB is about 300GB.
Rather get 4x250 or 4x300 disks and set up a hardware or software RAID so that it looks like one disk.
RAID 5 will allow a disk failure without losing data with the minimum waste of storage space.