You'll need about 20 MB of free disk space and 64 MB of RAM.
with a typical computer (such as a 2 GHz Pentium 4), you'll need to let SETI@home run for at least 2 hours per week (slower computers are fine but they'll have to run proportionally more).
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.