dual channel doesnt give as much performance gain in athlon XP system as you would see in a P4 system.

theoretically, yes, dual channel means 128bit as the bits are updated both at the rising edge and falling edge of the clock cycle
but in real life, the delta delays are longer and the CPU just doesnt work that well with it

thats said, dual channel ram isnt that much expensive
so its up to you
but whatever you do if you are overclocking, get low latency ram that can run fast and stable