What would be best to do, would be to buy a single stick and test it by itself, then if it works get a second matching stick.
Mixing memory doesn't always work, and even when it does it's usually a hassle, and with memory that old it may not work at all with any current memory. As for the speed differences, that doesn't matter too much, but it does matter a little if your PC wont work with a certain speed. Most of the time if the system BIOS only allows 2100 for example, and you buy 2700, it will just down clock to 2100 and run fine. That's not always how it works though, sometimes if you install incorrect memory it wont boot at all, so it's trial and error.
You're best bet may be to contact Crucial support directly and see if they have any compatible memory for this system, I see it's not in the scanner results, so they may not have any. You may need to find low density memory somewhere, like on ebay. Or you may be better off buying a new or even a newer used PC, instead of putting money into this old beast 
