Two new products have addressed this issue for me. One is the new Airport Extreme, which lets you hang a drive off the wireless hub, so all the machines in the house can send stuff to it. No more moving drives and cables around to do backups!
The second is Drobo. This is a smart disk enclosure that maintains your drive array for you. You pop in a couple of drives, format it once, and then throw data at it. It replicates the data automatically and guards against data corruption and drive failures. And it never runs out of capacity. When it starts getting full (there's a row of LEDs showing how full it is), you just pop in another drive and it automatically spreads the data around on it and makes the addtional space available. If a drive dies, it shows a red light in that bay, you just yank the drive and put in another one. If all four bays are full and you want to expand, it'll suggest a drive to replace (the smallest one, which is probably also the one with the most hours on it), you pull that one and put in a larger one.
There's no requirements that the drives match in capacity, speed or manufacturer. Any SATA drives will do. And during this drive juggling, all your data is still right there and available.
It actually spoofs the filesystem, interpreting the computer's read and write commands to determine which actual file the computer is dealing with, and stores it internally in its own filesystem. It can spoof FAT32, NTFS, and HFS+ filesystems.
It ain't cheap, but it sure is useful. You can pick 'em up at various retailers, but if you buy it directly from the maker at drobo.com and use the code EVJREHWIN you will get $25 off and I'll get a referral bonus. I elected to populate mine with drives that were in a sweet spot in the price/capacity curve, currently 160GB and 2 × 320GB.