I run two machines with my 4 alts on PC 2. My boxing software does software-KVM mouse/kb control so I don't have to switch hardware, I just mouse over to PC2 and click/type whatever I need. Primarily, I like 2 PCs so I can run my main + GameCam/Fraps/whatever and don't have to worry about performance issues. My boxing software also allows precise mouse-click broadcasting without having to run alts at the same size as my main, which greatly improves performance on the alts.

I'm about 99.91% PVE, but if I did I would still run 2 PCs due to the lag/performance issues in larger PVP engagements.

I also like having two PCs because I can do IM/mail/web/movies on my second monitor on PC1 and still do everything else.

Really, it just depends on how much disposable income you have for hardware (though you could spend about the same on two or three 80% PCs as you did on one beast of a PC) and what functionality is critical to your playstyle.