I want to confirm the memory > 3072 MB issue under Vista 64.
Before I came across this thread, I was dumbfounded to get ~19 FPS in vista 64 no matter what video settings I used (see machine specs below). I booted over to my XP-SP3 partition and was getting ~35 FPS (which is still WAY too low for my hardware but playable).
So, I limited RAM at boot to 3072 MB for Vista 64 as per original poster and suddenly got ~40 FPS @ 1440 x 900 4AA with fog enabled (still way too slow but better than under xp-sp3). Also of note, I just finished Divine Divinity, which of course supplied the engine for Beyond Divinity, and my framerate never dropped below 75 FPS @ 1440 x 900 with fog... looks like the engine was de-optimized!
Specs:
Q6600 @ 3.6 GHZ (quad core)
Asus P5Q Deluxe
ATI 4870 x 2 CrossfireX
4 GB Crucial Ballistics Tracer
X-Fi Titanium Fatality Pro
etc...
My thanks to the original poster for figuring this one out!
