There is no advantage to using the pc2 6400 ram in your case. The machine will clock down to the slowest ram installed. IE the pc2 4200.

In fact it might make it perform worse by doing so. The ratings have nothing to do with “quality” of the RAM. It has to do with clock speeds capable, timing and voltage required to use it. Start mixing them up and welcome to MR BLUE SCREEN if you under-volt or the machine cant handle the timings.

Do you have to buy what the manufacture recommends? Nope you can
buy anything you like. If you want the best chance of everything working as it should, take their advice.

Go to:
and use the Crucial Memory Advisor™ tool

also for a double check:

These manufacturers are the best IMHO and they will determine the types of memory applicable to yr system.

Then go to yr online retailer of choice and buy some. I recommend Crucial and Corsair as the best brands BTW.

Yes I see no reason why this wouldn’t work the companies just put in the lower grade stuff so they can keep prices cheaper