well.. i'm not expert in RAMs.. but i know a little bit la..
for dual channels actually it is system that make ur system faster.. 64bit X 2...
dual channel smth like we call we doing 2 things at the same time..

if u have 1 512mb onli, when u active task A, then ur prossecor will call ur ram start work.. then at the same time, ur active task B, then the prossecor will take the rest of the ram that left after use by task A to work.. means that 2 task r sharing 1 RAM...

on dual channel.. is u have to plug in 2 ram module.. so apply same example.. task A will use RAM A*, task B will use RAM B*... although there have alot memory left by RAM A*.. results each ram wont soo burden and work fast..

let us apply a very simple exmple,
if i plug 1x1gb 667Mhz ram into my laptop.. it will abit low efficiency than i plug 2x512mb 667Mhz although the memory size r the same for both situation.. this is b'cuz dual channel.. i can say that almost same as dual core(the concept)

about the "FSB" Front Side Bus... i can said that it like the road in the computer system.. erm.... or we can said that it is the connection between cpu and memory.. it wont duplicate with Dual core system or dual channel.. it fix..
For example.. ur prossecor only support 533Mhz, ur motherboard 800Mhz and ur RAM 667Mhz.. ur computer onli can run max 533Mhz FSB onli.. b'cuz it needs a good combination between ram, cpu & mother board..

some of u may hear ddr-667... ddr2-800.. for ddr2-800, its actually is running at 800Mhz FSB.. normally we will buy value ram.. thats 400Mhz, 533Mhz, or 667Mhz.. but there are othe hyper X memory that provide high performance memory.. normally their bus speed is at least 800Mhz.. some of then even 1066Mhz, that manage to push up the C2D power.. but of cuz.. those RAM's price also higher than value RAM..

for the laptop case.. i suggest u put in 2x512mb with 667Mhz.. thats the best system config.. or u can try 2x1gb(but costly.. rm800++).. 1 more thing, normally there is 2 slots in laptop.. u onli can do 2x combination.. immposible to put in 4x256mb..

this is all i know.. correct me if i'm wrong~

yup.. it sure will increase ur system performance.. but have to depends what u doing with it..

if u onli doing some normal task.. like surfing net.. listen music.. movies.. then actuall 512 is enough.. but u plan to play some games.. of cuz, its good to have 1g.. but for those who work as programmer or doing hardcore graphic thats need alot multitasking.. 2G or even 4G will onli enough.. if not u'll feel nth with 2G ram...

yup.. thats what i'm trying to said~