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You already know to get one with FW400 and whatever else it has (USB probably, SATA will be nice, also),
I'm assuming you meant "eSATA" there, Jim? eSATA is external SATA and Apple does not support it, so I wouldn't bother with it at this point. There is no indication they ever will support it either - they seem to have gone the FireWire route, even if nobody else is these days. I have an eSATA PCI-Express card in my Mac Pro, as I have one external drive with eSATA and it's lovely - except it's not bootable and the darn card is NOT supported in Snow Leopard. Grrr...how was I to anticipate that last year when I bought it? I'm not buying a new eSATA card when I finally get around to installing SL. Just have to use the USB 2.0 instead. It's not my main external backup anyway.
As for the fan/fanless debate - I have 3 external drives without fans and they're all in solidly built aluminum cases which seem to work very well for heat dissipation. One of them is going on 5 years old now, at least. Which is why one of my recommendations is for the Macally enclosure. Plus it looks like a little Mac Pro.
I wouldn't recommend a
plastic case with no fans though...Also, all of my drives have stands and are intended to stand up on the long side - I've got a wooden platform on top of my MacPro (which is in the corner beside my desk and more or less out of sight) and they sit there, all lined up, along with the cable modem and the Airport Extreme.
As for drives - as Jim says, the longer warranties are good, but do look at the reviews for the various models at NewEgg, even if you don't actually order the drive there. I try to aim for the ones with the higher approval ratings from users - the greatest percentage of 4 and 5 stars and the fewest one and two star ratings (which often indicate DOA drives or drives that died quickly). Things can shift quickly as the manufacturers come out with new models. I used to swear by Seagate - they offered 5 year warranties and seemed rock solid. Not any more - there are a lot of problems with their recent drives and I've been buying Hitachi and Western Digital (the "Black" line).
If you can decide on a capacity (given your current usage, 500 GB should be plenty, and is a good price vs capacity option) then we can make specific suggestions.
BTW - if you upgrade your OS to 10.6 (might as well, if you're interested) you can use Time Machine for incremental backups too. You could partition a backup drive to do both a cloned backup (still think SD is better than Silverkeeper, SB, from what I've read...) and the TimeMachine backup, which is nice if you want to go back to something you saved a couple of hours ago and then overwrote with more changes. You can't do a restore by cloning (copying) like you can with SuperDuper, but TimeMachine allows you to do a restore, using your installation disks.