I used Seti@home as a benchmark on my computers in the past! :)
The 1Ghz Athlon needs 7-8 hours for a work unit.
The 350Mhz G3 needs about 14 hours.
The 500Mhz G3 needs about 12 hours.
But i think it's a pretty cool idea, to use home-computers for a scientifical institute (such as S.E.T.I.).
Because of that (and because of the hope to find extraterrestrial life) i run Seti@home on my PC.
SETI@home and Astropulse are funded by grants from the National Science Foundation, NASA, and donations from SETI@home volunteers. AstroPulse is funded in part by the NSF through grant AST-0307956.