The "sections" in the Product Info page can be shifted around by editing tpl_product_info_display.php .
The ORIGINAL of this file is in:-
includes/templates/template_default/templates/
COPY this to your local drive and open it in a good text editor.
Look for the sections delimited by:-
<!--BOF .............>
(some php code here)
<!--EOF .............>
Find the BOF - EOF section relating to add to cart; CUT it from it's current position and then PASTE it ABOVE or BELOW the section (also BOF - EOF) where you want it to appear. It should go NO HIGHER than:-
<!--bof Main Product Image -->
(some php code here)
<!--eof Main Product Image -->
SAVE the edited version, and FTP it to:-
includes/templates/YOUR-TEMPLATE/templates/.
(I have done just this with several of my sites - see Showcase, below).