Go with the 4x512. The frequency may seem slower, but to compensate for this, you have every slot talkign to each other and transfering crap at a much higher rate. Frequency isn't everything, moving things however is. If you have the same amount of storage, which in your case is 1 gigabyte, and you can only move it across 2 sticks, you'll be slower as oppossed to moving it across four sticks.
Trust me, go with the 4x512, you'll definetly like it.