From a gaming perspective:
At one point it might have been considered a downgrade, as most games only used 2 cores and a dual @ 4.0 beat out a quad @ 3.6... however, just about all games now benefit from quads, so it's definitely a wise move to jump to a quad even if it is just a q6600. Ideally though, you'd want to upgrade to a q9x50.
From other uses (video encoding/rendering/multitasking/etc) perspective:
Quads = better. No further elaboration really needed.