In the 2000+ the oil pan gasket is not as common as the 92-99 style. In 92-99 there was a plastic gasket that acted as a windage tray and didn't seal well. 2000+ had either a rubber inlaid gasket or was engine bonded with goop to the motor w/o an actual gasket and the windage tray is part of the pan.
A very common and usually overlooked leak is the rear engine cover. It'* impossible to see, impossible to repair w/o pulling motor/trans and usually seeps.
To diagnose the leak, I'd hop back under the rear and look up with a light at the engine. Like mentioned in an earlier post, the valve covers are quite common points of leaking. If you see oil going up the block/head, you will want to change the valve cover gaskets first.
00 Regal GS: All kinds of cool stuff.