Ðave wrote:

Win XP Pro & Vista Ultimate

Which is your primary, gaming OS? Please say XP!

You realize, Vista's SLI implementation is not ready for prime time.
Also, SLI profiles are application specific and the fact that you do see
gains with SLI in some apps, but not others indicates your configuration
is correct.

Time and driver revisions are the likely solutions, however, demos
aren't games and if they do not take advantage of SLI currently, they
may never.