Requesting help with the placement of dimms in relation to ram performance?

Does the placement onto the memory cards of dimms of different capacities/size matter when it comes to performance ?

I want to know if it matters how I install a new pair (2 pcs) of 4gb dimms in a machine currently with 8 x 1 gb dims?

IF it matters, which exhisting dimms should I replace for best performance?

Should they be paired in slots 1 and 2 of the upper (or lower) memory card, or separated into slot 1 of each card?

Is there a preferred slot number for dimms of a larger capacity than the rest.I hope I am being clear enough in my description.

Thank you in advance for any advice.