Upgrade has now been completed and it has enabled me to install ML 10.8.2, which is running OK, apart from:
1) no wired internet (because AFAIK there's no 64-bit driver for the onboard Broadcom 5751 NIC). I was looking into the possibility of a workaround with VBox (details on another thread in this forum) using a Netgear USB stick I already had lying around, but that was a non-starter because my CPU does not support virtualisation. Problem should soon be solved with the imminent arrival of another USB WLAN stick (so small it looks like a button actually) which has drivers for Windows and Macs (10.4 to 10.8) and which cost 10 bucks including postage.
2) no QE/CI as there's very little ML support for GMA950 but I'm not too bothered about that. Graphics aren't amazing (I would like a bit more choice of screen resolutions) but I guess I'm pushing the limitations of the hardware of my little GX620 to the limit. Can't ask for more really as I am now able to install some of of my favourite programs that weren't possible before on Leopard and SL.
If anyone would like to see the photos I took of every stage of my CPU upgrade, just shout and I'll upload them here.Go to the full post