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!

Seeing a few startups' websites fall over says precisely nothing. They could have been on $20 VPSs for all we know - a twenty dollar VPS being something that is possible with OSS. Or maybe not well tuned - since the vast majority of startups use open source software, it's only natural that you will see a disproportionate number of sites using that software fail.

Anyway, I'm glad your technology choices are working out for you. However, you must understand that a $2k per box markup would be a stroke of death for many lower-margin startups.