RAM for Gigabyte GA-851 ML

Are you sure you got the model right?  I looked at crucial's site and kingston's site and that board is not listed - even google doesn't give me any links.  In most cases with DDR RAM, using a higher speed module will work, but there's a small chance of incompatability.  Crucial will guarantee that their memory will work.

I looked in Google and got nowhere which made me wonder if I  copied it down wrong. PC was a Packard Bell 2002 2.19 P4.
Haven't heard of Crucial memory - will try looking for that and when I visit again check the MBD name more carefully.

Is there an easy way of the client checking - without using something like Everest?
She is not very technical but something I could talk her through.

Having looked on the Gigabyte website it could be 8SI rather than 851!!

This could be it: http://www.ocmelbourne.com/reviews/8siml/8siml02.shtml

If she is willing to open up the case, the name and model is usually stenciled in white along the side of the motherboard.  Sometimes, it will appear on the bootup screen.

Its a Packard Bell iMedia 5090 (not listed as an option on the Crucial memory checker)
P/N P480411101

Unfortunately, there's not much information on what motherboard Packard Bell uses in their models.  For example, the 5091 just has this: http://www.packardbell.co.uk/products/node1601.asp?PartNumber=p660400601

if the system is Internetable... you can try and use the CRUCIAL SCANNER>....   http://www.crucial.com/

I have had some success with it... esp on oldersystems

please give us the right model of your motherboard:
cuz i found:
ga-8siml-h-p(rev 4.0)
ga-8siml-h-p(rev 4.1)
ga-8siml-h-p-c(rev 4.0)
ga-8siml-h-p-c(rev 4.1)
ga-8simlhp-tw

so which one is yours?