What kind of cable do I need to connect my laptop to my stereo receiver? Would one of those "Y" cables do the trick? Or is there something that would yield higher quality?
Well you could do it using airport express, which would allow wireless streaming via 802.11g and an audio out port.
Since your powerbook doesnt have an optical (S/PDIF) out, the best quality you could get without extrenal hardware would be through the headphone jack, using an RCA (red/white) to headphone minijack adaptor.