| how much is enough
ok well start off with with a little history . i remember back in the day when all the computer techs said that all you would ever need was a 133MHz processor, 1.8 gig Hard Drive and 32 megs of ram. then again i also remember upgraiding from a 386 to a 486. any ways.
ok here is the history lesson in short. back in the day, before cell phones. the whole memory issue was simple. the rule was the more the better. but now that is not necessarily the case. memory comes in differant speeds, and differant latency. first off you need to konw what you are going to be doing on your computer. if it for typing the occasional Christmas letter, and to check your e-mail one a day, then 512megs will do you just fine. most of the newer computer come with 1024. other wise known as a gig of memory. verry fiew people need more than a gig of memory. now it also depends on how you have your memory set up. it is also important to know that you should alwase use memory that is the same size and by the same manufacture. it will not hurt to use memory of differant sizes or manufactures, but your memory will not work to it full potential. and then that is wasted money.
why you want 2 sticks in stead of one
if you are running a single 1024 stick of memory, or if you are running two 512 sticks of memory. two 512 sticks, your computer will run faster than if you are running one 1024 stick. yes the amount of memory in the two are the same, but the pipeliine. on the two 512 sticks is twice as much. the little pins on the ram sticks are how the informations enters and exits the ram. now think of it like this. think of those little pins as lanes on a highway. the more lanes you have, the more traffic can move in one dirrection at the same time. so if you have twice the number of lanes it moves considerably faster. (not twice as fast).
latency
this is more advanced, and most people will have no idea what you are talking about here. latency is how much time your memory spends doing nothing. it is the amount of time between when the memory sends the informatin out, and when it gets new information in. there is more to it than this, but this is the simplest that i can make it. so any ways, as you would guess, the lower the latncy the faster the memory. latency is displayed in one of 2 ways. either 3-3-5, or 2.5-3-3-8. now the numbers will vary between manufactures, and the quality of the ram. but a 3-3-5 would be faster than a 4-4-8.
speed
memory also comes in differant speeds. pc2700, pc3200, pc3500, pc3700,and ect. of courst the bigger the number the better. memory also comes in speeds in terms of MHz. normally around 400MHz but can go upwards of 500MHz. | 
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