quad channel : best memory configuration

hi,

I have a mac pro 2.66ghz and want to be able to run with quad channel memory.

I have 4x512MB and have got hold of some 2x1GB modules. I'm not sure they are set up to run quad channel. The question I'm asking is how to position them in order to achieve the best performance?

they are currently positioned like this:

riser A
dimm1. 512MB
dimm2. 512MB
dimm3. 512MB
dimm4. 512MB

riser B
dimm1. 1GB
dimm2. 1GB
dimm3. empty
dimm4. empty

is there a better position?

hope you can help me out here.

best regards

dajaj