Question: Which is faster, two 256 MB modules or one 512 MB module? Answer: it depends. If your system's motherboard, chipset, BIOS and CPU support something called "dual channel", then the answer...
You reduce the amount of additional memory that can added without replacing the 1st module. Eg. 3 slots and 2 64's gives a possible addition of a 128 : 64+64+128=256 3 slots, 1 128 leaves room for 2...
As the memory speed is limited, you essentially double the available speed by having two modules, as they run in parallel. The efficiency is generally increased by 20-30% by having two 512mb modules...