Would a Core 2 Duo E6700 based machine be much faster than a P4/3.4 571J
based machine, assuming all other components are similar (except memory)?

Worth the upgrade?

Not noticeably.
However if you do alot of Photoshop or video processing and future programs
are double threaded the changes should be significant.
The only intensive tests of the Wintel core duo have been the Extreme
Edition which appears to be a stellar performer at an astronomical price
point. The middling core duos, such as showed up first in the Mactel
machines, were not such stellar performers and have yet to be extensively
tested in a variety of Wintel hardware.