I worked at Dell and got this one all the time. The 8100's use Rambus memory, otherwise known as RIMM's. Yes, they have to be paired. Here are the specs:
Memory
Architecture RDRAM
RIMM sockets four
RIMM capacities 64-, 128-, 256-, and 512-MB RDRAM
Standard RAM 128 MB (minimum)
Maximum RAM 2 GB
BIOS address F8000h
So, basically he will have to buy two sticks of memory, he can not just buy one. Also, be sure he updates to the latest bios before installing the new memory, it helps alot with the 8100's.
You do not necessarily have to buy pc600 (i doubt you can find it)...you can probably go with pc800 and it will run at pc600 speeds.