The simplest and, in many ways, the best way to deal with an underpowered web site is to throw hardware at it. PCs are the cheapest way to buy MegaFlops, and TCP/IP connects them together nicely. All that’s needed to make a server farm is something to balance the load around the PCs, keeping them all evenly up to the collar, like a well-driven team of horses.
There are expensive solutions: Cisco’s LocalDirector, LinuxDirector, ServerIrons, and a host of others.