When do I upgrade my server?

As with all things computer, I always fall into the trap of wanting to upgrade. Is there any way, in the style of Netcraft's What's That Site Running, where I can find out how much processor / RAM / etc. various sites are using?

My server (in my basement) is an 866 Mhz PIII w/ 384 M SDRAM and a 40 G IDE HD which is undoubtedly 5400 RPM running Windows Server 2003, Apache 2.0.55, PHP5, MySQL 5, and--the good part--a 15 Mbps full-fiber dedicated upload. But everyone says that's more than enough for my site, despite its database dependence. I tend to imagine huge server racks running various sites, but PHPBB.com just put up a picture of its rack, showing its running off ONE 2U server. So what does it take to make my site fast? How much hardware powers this site or others?