I just finished upgrading on old pentium II system running windows 98 to a pentium 4 motherboard. There is a nasty bug in Win 98 and Win 95 where most windows installations will crash when they startup with a "General Protection Fault" for NDIS if you try to install a CPU that runs at 2.2 GHz or faster. This wasted many hours of my time since I simply assumed that Windows was having trouble recognizing the onboard lan (which seems happed fairly often) so I wasted a lot of time with bios updates, trying several different versions of the lan driver etc. etc. Instead, none of this mattered -- it was just that the CPU was too fast! Here is the link to the Microsoft KB article 312108 so you don't go through the agony I did if you ever upgrade your old machine like me:
http://support.microsoft.com/default.aspx?scid=kb;en-us;312108
http://support.microsoft.com/default.aspx?scid=kb;en-us;312108