it is annoying that the demo will be released in September (or even later).
My problem is that I do not know the prequels and also just saw a video of the game.
Playing the previous games would not help as far as combat, since the controls have changed in D2:ED. The demos for them should still be around somewhere, though, which would give you an idea of the style and atmosphere that will most likely be carried over into the new game (both were designed to be fairly linear, though the DD demo is more open and has more NPC interaction, etc). Also, while not necessary prerequisites for D2:ED, playing the first two games (or at least Divine Divinity) would certainly help hold you over until September (it is still possible to find both online (eBay, etc) and Beyond Divinity can be downloaded from GamersGate.
I have also had to wait awhile after a game's release to get to play a demo. In this case I am willing to 'gamble' based on the previous 2 games and what I know about D2:ED, but couldn't say that about many other new games (old/cheap games I am a little more willing to gamble on based on reviews from trusted sources).
If the English PC version goes gold the same time as the XBox versions, then (depending on how long it takes to make the demo compared to the lead time it takes for manufacturing, packaging and shipping the games before release) there may not be too long a wait for the demo. If not, at least there should be lots of help and information available by the time the demo is out, from those who didn't wait.