Here's a PDF from Sonnet describing how to determine if your Sawtooth can use dual processors:

http://www.sonnettech.com/publicfiles/pdfs/pdf_onlinedocs/english/mac/encore_st_duet_2g_add.pdf

Mine was compatible and now has a dual G4/450 in it which cost about $40. Best upgrade I've done for it so far. While there are faster processors available, I could afford this one.