Compressor generates 853.3 x 480 QT from anamorphic DV sequences. That's the number I have been using when I get squeezed QTs from various sources.
It would be 864 x 486 if you were working with CCIR601 anamorphic.
Your clients PSDs should have been composed with widescreen in mind. If they weren't you'll have to decide to either stretch them to fit, leave them unstretched which wouldn't work if there were elements that went from edge to edge, or recompose them in Photoshop at the proper aspect ratio.
ak
Sleeplings, AWAKE!