Maybe I misunderstand your question, but the default block size is 2048 bytes, not megabytes. Even then, from what I recall you're only writing zeros, so you might not even have to store the whole block in RAM. You can easily pre_clear drives with 2GB of RAM. I was doing 3 at a time on my old machine.
Crap. No, you didn't misunderstand, I did. Bytes =! megabytes Oops.