I've read mention of the ratio and wanted to know exactly what is meant by 1:1, or 4:5. How does one change this ratio and what is the advantage/disadvantage of doing so?
In current Intel CPUs, RAM is on a memory bus controlled by memory controllers in the MB chipset. The memory controllers are connected to the CPU via the Front Side Bus (FSB). The ratio you mention is that between the memory bus and the FSB.
Since the data must pass through the FSB, the FSB throughput limits the amount of data transferred to/from the CPU. To allow as high a throughput as possible, the memory bus should have a throughput at least as high as the FSB. The FSB is quad-pumped, meaning that data is transferred at 4 times the bus speed. For most C2Ds, FSB speed is 266MHz, for 1066MHz effective data throughput. DDR2 memory transfers data at only twice the memory bus speed, thus you might expect to need DDR2-1066 memory to keep up with the FSB. However, dual-channel mode doubles the memory throughput, so memory run at DDR2-533 in dual-channel mode can provide 1066MHz data throughput. This would be a 1:1 ratio. Higher ratios (faster memory bus speeds) won't significantly increase the overall performance, because FSB throughput will be limiting.
For most CPUs, the ratio between the FSB speed and the internal CPU (core) speed is fixed. Thus, if you increase the FSB clock speed, you automatically increase the CPU core speed. This is overclocking. If cooling/voltage/etc are modified so that the CPU runs stably at higher core speeds, overall performance can be boosted substantially, because (1) the core is running at a higher speed and (2) memory throughput is boosted by the increased FSB. Faster memory bus speeds (e.g. DDR2-667 or DDR2-800) will be needed to keep up with an overclocked FSB.