You cannot mix different types of memory in the same machine. If you intend to get more memory you would need to get one or two sticks of PC100. You can get 512MB in one stick. PC100 is SDRAM whilst PC2100 is DDR and different. A good guide to memory stuff is here click here and I'm sure some of the more knowledgable "forumers" will be along soon
BTW. If you were going to order from there rather than Crucial, don't bother with the generic stuff, get branded or named memory. I would usually favour Crucial but sometimes it's not possible to obtain such dated hardware.
Single and double sided RAM is VERY important f you have an older system - which that *may* be.
It's more then whether chips are mounted on bot sidea of te module - and sometimes (a bit more accurately) referred to as low and high density.
The actual difference is in the number of address lines used to talk to the modules by the system. Low density modules only have 15 address lines - which is all some older system s have. High density uese 16 lines - so what happens if you put a HD module into a system that only supports LD, typically you only see half of the RAM capacity.
A system of that age *may* only accept single-sided (low density) modules - so it IS very important you get the correct ones.
If your system is capable of using HD modules, then it can accept both types - which is why some people never see any problems with either type.