Yeah I say you will want more space eventually, and consider downloading TV shows, movies (legally probably coming very soon on iTunes, and those files are probably going to be relatively big), and a ton of music, all this stuff will end up taking up a ton of space, not to mention if you have a really nice digital camera, those files can get huge. Games are getting bigger too. It's always good to plan ahead.
Plus consider that all the apps and system take up a good amount of space as well, and they're only going to get bigger in the future as Apple adds more templates.
An external is a good idea, but it's also nice to have a lot of internal space on your startup disk, that way you don't have to worry about where you're saving files, and you can keep everything organized in your user folder where it should be.
I suppose in the end, though, I'd have to agree with SmurfBoxMasta and go with the 250 internal and get an 500 gig external. Maybe one of those nice LaCie's.
You may also just want to start out with your 250GB internal and buy an external later on when you see your space start to run low, that way you'll end up saving money because external drives probably will have gone down in price by the time you fill your internal up.