Dual monitor and performance

Hi,
Has anyone here measured the amount of resources it takes if the clients are using dual monitors?
Are there any guidelines on how much bandwidth and server capacity is required at different screen resolutions? I have many users how use dual 24" monitors with their thin clients and they experience a slowness in their sessions. It was not as noticable when they used one screen.

Thanks for any imputs.

Regards,
Boffen