Hello!
Thank you both so much for your answers! Sorry for the delay! I wanted time to think! Northbridge was not something I had actually heard of before, but I have now, and it certainly seems to be the right thing. It has probably been hot for years. I shall just ignore it, and you have quenched my curiosity! Thanks Pebbly! Topic one resolved!
Again, thanks Pebbly for the excellent link! I read it, and learned a lot! And thank you, Joan, for your excellent contributions!
I have got another question now! *worry all around* I ran the crutial memory scanner:
Memory upgrades from Crucial.com - Determine My Memory Needs
It offers memory at 1.8V and 2.0V. This thread says very definitely that 1.8V on 2.0V is fine:
will running 1.8V ram at 2.0 kill it? - Overclock.net - Overclocking.net
But I don't know what I have! I assume that 2.0V will not work on 1.8V. I looked in the Dell manual, but couldn't find anything about it. I assume that I am on 1.8V, but not for any particular reason.
If I need 1.8V, that gives me this:
4GB kit (2GBx2), 240-pin DIMM, DDR2 PC2-6400 upgrades for Dell XPS 420 Desktop/PC, CT1167040 from Crucial.com as I don't want to go for the slower speed. £10 for not losing memory speed is money well spent - it is only £10.
However, if I can use 2.0V, I have far more options, and even some faster memory like this one:
4GB kit (2GBx2), Ballistix 240-pin DIMM, DDR2 PC2-8500 upgrades for Dell XPS 420 Desktop/PC, CT1198084 from Crucial.com
I shall contact Crucial to see if their scanner actually accurately gauges speed properly, and I shall hunt around for a motherboard manual.
Thanks again!
Richard