Latency is the time it takes to access the RAM, and bandwidth is the amount of RAM that can be theoretically accessed per second.
Athlon 64s have an on-die memory controller, which lowers latencies significantly. Bandwidth is also high with the dual-channel DDR platform for Socket 939 processors.
Generally, increasing the memory clock increases bandwidth. But you can only increase the memory clock to a certain point with certain latencies. For example, my TCCD memory does 2-2-2-5 latencies at 200MHz, but I have to lower the latencies to 2.5-3-3-5 at 260MHz.
Is bandwidth or low latencies better? That is ultimately up to the task at hand. If you're thinking of a particular application or game, benchmark it with both settings and decide.