Mac Pro Ram Configuration

I have 6 GB of RAM installed in my Mac Pro, configured as:

512 MB - Riser 1 Slot 1
512 MB - Riser 1 Slot 2
2 GB - Riser 1 Slot 3
2 GB - Riser 1 Slot 4
512 MB - Riser 2 Slot 1
512 MB - Riser 2 Slot 2

Is this the best configuration? Would it be better with all 4 512 MB sticks in riser 2? Do you think there'd be any noticeable performance gains?

Thanks!

I thought it had to be installed with the highest capacity DIMMs installed first. I would install the 2GB DIMMs first anyways. And I don't think you could leave slots 3 and 4 open on the first riser if you have DIMMs on the second riser.