The MacBook Pro will run fine with either a single memory slot filled or with both memory slots filled with two unmatched memory sizes (such as a 512MB DIMM in one slot and a 1GB DIMM in the other).
However, if both slots are loaded with an equal amount of RAM (such as two 512MB DIMMs of the same type), you can take advantage of the system's dual-channel memory architecture for an additional performance boost. With a dual-channel memory interface, both banks of SDRAM can be addressed at the same time, enabling MacBook Pro to reach a memory throughput of up to 10.7 GBps.