starting small with growth in mind

Hello all,

I'm working on a web site that might totally flop or it might explode to be the next facebook/flickr/digg/etc. Since I really don't know how popular the site will be I don't want to spend a ton of money on the hardware/hosting right away but I want to be able to scale it easily if it does grow rapidly. With this in mind, what would be the best approach to launch the site?