...you must be talking about RDRAM. With DDR, if you want to have dual-channel, then you need to have an identical pair of memory modules, but they will work alone, as well
I would recomend going to kingston.com and there you can put in you computer brand and model and it will tell you what type of RAM you computer is compatible with. I would say to get 1GB of RAM since it is a pretty good amount these days.
Installing RAM is easyer than buying. Just place the sticks in the RAM sockets, they only fit one way so you cannot put them in wrong. When its in close the two prongs on the sides of the sockets. If the prongs dont close make sure the RAM sticks are fully inserted.
DDR RAM also should be installed in pairs of the same amount of memory.
I hope this has answered your question.