AVR-Chat

> I will not recommend this kind of circuitrs unless for cost
> reasons. They tend to be quite noisy.

Well, Nationals simple switchers aren't too wonderful, but unless you
lay out the board with no consideration for the circulating currents,
they certainly won't interfere with the AVRs operation.

Noise is something that you minimize in switchers by paying attention
to the components you are using, and to the layout itself.

Unless you have something fairly sensitive in analog going on, an SMPS
is a good choice.
Keeping the voltage on the bus high is a good way to minimize i^2R losses.

Be sure that you are taking into account the interaction between the
supplies though, (this would apply equally to linear supplies).
When unit X draws current, it will affect the current available to the
other units near it on the bus.