I don't know much about PVM, but from the looks of it, you wouldn't notice much of a difference for games, games are more your video card and memory, which wouldn't gain anything by going over a network to other computers.
PVM is a good solution for big mathematical calculations, and number crunching, like if you were running decryption algorithms or something.
Someone might be able to tell you better, that's just what I got from looking at a few sites just now. (I hadn't heard of PVM before, but it looks pretty good. Maybe I will set it up for encoding Divx movies, but even there I don't know how much I will gain.)