PaulS:
The reason you have been suggested to use the ethernet shield is that it would be much faster than USART.
Have you verified this? The Server and Client classes bear a close resemblance to the Serial class. I don't think that the ethernet approach will be any faster.
No, I haven't looked at the classes or the shield, my apologies. I just assumed that the atmega together with the ethernet chip in the shield would run fast enough to to offer a significant speed increment compared to USART. I should have not jump into conclusions.