You can use these for virtual hosting, but you're better of getting a white box pc/server for that. You can put linux as well as freebsd, and windows on it.
With the sun you can load linux for sparc and solaris.
Unless you really need to use Solaris, for something like ONE, iPlanet, Oracle, etc.. don't waste your money.
Those fire servers are great, may be you want to offer Sun hosting to die hard java developers that need a real environment to deploy there j2ee stuff on. Most general hosting clients won't have a clue what to do with them.