A general overview:
If you have a minimal Debian installation, for example (something that can handle a small hard disk and amount of RAM fairly well), you can first optimize its RAM usage by disabling extra consoles (normally, IIRC, it has six that you can switch between with CTRL+ALT+Fx; you should only need about two or three for a single-purpose cruncher). Then shut down services you don't need (no sendmail, apache, ftp, telnet) and only leave running the bare essentials (ssh, and possibly samba if you need it for the administration of the SETI client) running. Then start SETI from your init scripts.