The connection of the peer is more important: this will affect the speed more directly. The bitrate indicates the quality of the file, for song files: a bitrate of about 126 means an ok sounding song whereas a bitrate of 300 will get you A quality sound. Higher bitrates mean larger files (as less stuff is cut down during the encoding) which also means higher waiting time for the download to finish. In that aspect, bitrate does influence speeds.
I think in Kazaa, bitrates reffered to the actual effective transfer speed of the host. That's probably why your bro-in-law told you to look for better bitrates.
I don't know much about the WMP as I don't use it all that much (sure I'm on Mac, but even so...)

EDIT: Oopsie, I didn't know you were also typing your own post at the same time Lord. Sorry

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

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