I wouldn't exactly recommend running the screensaver on a Pentium unless you got a graphics card capable of handling OpenGL else it'd be dog slow.
However, if you go back through some of the posts in the last couple months you;ll find some people actually have run it on older systems using the ASCII (Command-Line) client. You can actually complete a reasonable amount of work with your Pentium say as versuses SETI or Folding which could take days to complete one work unit.
I think the general idea is because of the nature of the project, throwing a lot of firepower at it in attempting to create as many structures as possible (specifically up to 1 billion) to see how accurate their prediction algorithims are. So its actuall really a preference so they don't sit here for the next 6 months running the current phase.
There's also the idea that its designed to run on your spare CPU cycles, and based on that idea, it would greatly extend the time to return work and possibly have a greater impact on the user's system performance. Although truth be known, I think Howard has been quietly and unofficially curting those with powerful machines or access to lots of systems. It seems like there are a lot more heavy hitters on here than some other projects out there... that's just my opinion though. =)
Its not necessarily a bad thing... its alternative approach to the SETI@home lets get everybody and their dog and cat (yes even cats!) to run this on their systems is a good idea for a younger project with a smaller base of users.
TTFN,
RuneStar½