I had a similar predicament with my laptop, but being a cheapskate chose to buy a USB powered external hard disc case (eBay - about £8 think) and then make my own. I bought a 60Gb laptop hard drive for about £80, mail order from one of the ads in a PC magazine and the whole thing went together like magic in less than five minutes (not even a single screw!).
I like the small size of the drive, it's very slim and fits easily in a shirt pocket. The fact that it's powered by the USB port makes it easy to use as well. I've found it to be pretty quick (USB 2) and not noticeably slower than the internal drive on the laptop. It doesn't need any drivers or installation either (Win XP), just plug it in and it works.