Either a NAS (Network Attached Storage) if you have a broadband WiFi Gateway/Router with a spare 10/100 RJ45 port, or a USB 2.0 drive. I'm currently playing with my 2 Buffalo Technology ones (1.5TB NAS LinkStation Live, LS-CH1.5TL) with an encrypted, USB 2.0 drive (1.5 TB DriveStation, HD-CX1.5TU2) hanging off it, as I've been impressed by their WZR-RS-G54 bridge/gateway/routers' extensive and easily configured setup interface, of which I have two. If you're interested, go to
http://www.buffalo-technology.com/products/ and research them, then eBuyer.com for the cheapest I could find.
HTTH
P.S. the NAS has Apple AFP support and the USB 2.0 drive is FAT32-formatted.