External Hard Drive Recommendations

Any pointers on choosing a hard drive? E.g. Bus powered or power brick, 3.5in or 2.5in? Manufacturers to avoid?
Also any ideas on how best to set up a Windows partition - Emulators or Boot Camp or whatever?
I know I'm being a bit lazy and will need to research this a bit for myself, but a few recommendations just might stop me from putting it all off for another month/ year!