ORAC,
Is your power supply at least 300W? It could be something as simple as that : by adding additional hardware that requires power could be causing the problems. (N.B> AMD recommends using power supplies endorsed by them, but it's probably not a must, just a should be.)
Out of interest, are you having any conflicts reported with either "SB16 emulation" or your LPT port ? I had this the first time round, but everything is happy now.
Good luck
Sq