Re: Serena's level against Dementieva won't cut it against Venus.
Serena has a few gears in the mental game department. She has shown it many times that after a weak performance in one match she can deliver an extremely focused and crushing performance. She had some of those in AO finals, big matches against Mauresmo and Henin and most recently in the quarters against Azarenka. She gets mentally drained and can not keep it for long, multi match stretches. She definitely didn't have it against Dementieva. She didn't see the ball as sharp, didn't adjust her steps, mistimed the hits and had to rely on guts and power to get her through.
But, with the big occasion she might just step up and start "treeing" for a whole match. If that happens she will break Venus spirit within a set and put too much pressure for Venus to handle later in the match.
My money (if I were a betting man) would be on Serena in 3, after a close first set, cruising in last two.