Personally I have never given a presentation to a VC however I have had to give presentations to collegiate audiences and I can tell you that standing helps in those situations. Indeed, I have never even seen any professors sit down while lecturing. As is said elsewhere in this thread, body language is restricted while sitting. Body language can capture the audience's attention and put an energetic charge into your presentation which can excite your audience.

If you hit your audience with a series of dull PowerPoint slides, be sure to bring lots of coffee to help them stay awake. In college some of my professors would do nothing but flip PP slides and drone behind a pedestal. No movement other than to click the mouse. Oh the torment of those classes. Never underestimate the potential destructive power of PowerPoint to otherwise good information.

Recently I was introduced to a group that actually teaches how to do public speaking. For all those interested, it's called toastmasters. You can find them at http://www.toastmasters.org/ Some of their members include people such as senators, prime ministers, and lots of CEO's so it can't be too much of a waste of time and money.

I have not had the time to try it out but I believe that it will be worth a look shortly.

Regards, Derek