Don't buy an iMac as you can't add more drives to it. If you even want to upgrade the hard drive, you'll void the warranty as it involves removing the glass and then the screen... can you say dumb, dumb, dumb, dumb, dumb?
Get a Mac Pro instead and to avoid ripping your hair out, if you do install Windows, make it XP, as someone here also said, Vista is really bad right now. Leopard is also going through it's growing pains (Tiger was perfect) but nothing like Vista. ::I::
I guess it depends on your needs and what storage capacity you can live with since external drives are more than twice the price of internal drives.. You can't install the Windows partition on an external drive, so I would get the biggest internal drive possible but for what Apple charges for larger drives, I'd consider the Mac Pro with a Dell Ultrasharp screen. I find the iMacs don't offer enough expansion like video cards, PCI etc. And like I mentioned, I find the Dell screens better than the Apple ones.
I do buy iMacs for labs here at work as students don't need all the bells and whistles and friends also like them, but they do have their limitations and I like options. ::I::