First off, I'm not a Microsoft salesman, so please take anything I tell you as the experiences of one person, and weigh it against your own.

That being said, one box on the MS stack can handle a lot of capacity. Enough that I'm constantly amazed to see other people's startups folding under the load of, say, a single slashdot article or a few thousand people kicking the tires at once.

The box that Twiddla lives on has seen some pretty heavy days, and it's never had its CPU over 5% in anger. Considering the number of paying users we'll need before we need to add a second one, well, I'd be delighted to pay another $2k out of that stack of money!