1. Build tagged, socially networked, Web 2.0ish, Ajaxy thing

2. Get 500,000 users quickly (and have it actually work)

3. Profit



500,000 users at an average of 100 front/backend requests daily

= 578 requests/second.

x 3 (because your traffic won't be evenly distributed)

= ~1700 requests/second.