Everything has not been so peachy in dual core processor land since my last post. Although it seemed to be working fine to start with, after a random amount of time suddenly the mouse pointer and keyboard would become very unresponsive - lagging behind my mousing or typing by a second or even more. The only way I've found to correct the issue is to reboot. After some experimentation (I at first thought the problem was related to having both on-board sound and a sound card) the only (apparently) working solution I've found so far is to turn off APIC altogether with my boot options:

kernel /vmlinuz-2.6.17-1.2157_FC5smp ro root=LABEL=/1 rhgb quiet noapic nmi_watchdog=2 noexec=off

This sends all interrupts to one of the cores, which isn't ideal but not noticeable in normal use. I'm not sure if I still need the nmi_watchdog option now that APIC is off, but it doesn't seem to be doing any harm. One option I didn't get round to trying yet was to change the BIOS setting for 'MPS version' from 1.4 to 1.1, some laptop users reported that they couldn't boot up unless this was set to 1.1 but I could find no specific information for my ABit AV8 motherboard.