The My Books from Western Digital are pretty good and you can get a 160GB one for about 70 dollars.
Either that or you can buy an external enclosure separately, buy a 3.5inch drive and make your own. The advantage to this is you can easily swap out drives if you want to upgrade later, keep close watch on what type of enclosure you buy and the hard drive though. There are the EIDE enclosures and drives and the SATA enclosures and drives.
There are also 4 different types of connections right now, USB1, USB2, Firewire 400 and Firewire 800, USB1 is all but out of here, but you may want to make sure that the enclosure or external hard drive is USB2. USB2 and Firewire 400 are nearly identical in speed, so it's your preference. Firewire 800 is very fast, I've clocked my firewire 800 external at almost the same speed as my EIDE internal. It's probably important to note that unless you have a pretty new media PC, firewire does not come standard with PCs, you'll need to buy a card and install it, also firewire 400 and 800 are different in connectors and prices, also if your PC currently has USB 1, then you won't see the speed in the USB 2 device, so you'll need to buy a USB 2 card to install.
Either way you go, I'd be careful about relying on one hard drive as a backup solution. The drive will eventually fail. It's a great temp storage area until you copy to DVD/CD, but not for a permanent solution. If you want it more permanent, you might want to invest in the RAIDed externals.