It'll be running at the speed it's set to in all probability - unless it's set ti SPD detection. It "should" be running at 66MHz so as to allow the slowest one to function correctly. Sometimes 66MHz chips can be run fine at 100MHz - and many older boards were set to that as a "compromise.
However if it's running OK, I wouldn't change things unless you want to add more RAM. SDRAM is not cheap now - and as it's an older system, and it has got a reasonable amount of RAM with presumably a not blindingly fast processor - if it was my client I would advise them to do nothing - but recognise the performance limits of the system as a whole.
Yes you are right the processor is only 850 mhz but she would like to increase the RAM size but perhaps not now when I tell her that SDRAM isn't cheap! I thought that as it's old architecture that the price would drop like what happens with other PC technology like graphics cards?
If you do buy any more RAM (and I still stick to my advice of don't buy any unless you want to up the overall quantity)....
...then be aware that you might have an issue with "single-side" (low density) RAM or "double-sided" (high-density) RAM. Some motherboards around that time have such issues (not all) - so it pays to buy from somewhere that will give you specific advice on this and/or guarantee an exchange in case of problems.