I have an Asus P5E3 Deluxe, and everything works great with one 8800GT using EFI strings.
When I insert another 8800GT, the system gets stuck during boot on: "IOAPIC: Version 0x20 Vectors 64:87" Normally the line that follows this is "ACPI: System State", so I assume it goes wrong there?
I also tried inserting another card (8400), but it has the same problem, gets stuck on IOAPIC. I have 3 PCI-e slots, and tried all combinations. Always the same.
This happens with a DSDT I found online for the board. I made my own DSDT by extracting it in windows, and patching with DSDT Patcher (no special hacks, just regular patch). Still same problem. When I boot WITHOUT DSDT, it gets passed the IOAPIC line, but gets stuck on something else a little further. Since the DSDT influences the problem, I'm hoping it can be fixed with a better DSDT, but I have no clue what to do.
I tried adding PEG1/PEG2 sections in the DSDT with the memory addresses, as described by some articles. This compiled, but didn't change anything. As soon a any DSDT is present, it gets stuck on IOAPIC if there are 2 cards inserted.
@aquamac: I noticed you once did a project with a P5E3 Premium and multiple cards. I'm hoping you might know how to fix this? Or perhaps you have a DSDT that might help?
I continued on the path without DSDT, and has some luck. While trying some EFI strings I found on other forums I found one that gave me a kernel panic on NVDAResman. After googling the problem a lot of sites suggested to delete GeForce*.kext and NV*.kext.
It booted but with only one card working and no hardware acceleration. I tried other EFI strings, but things were the same. I decided to read all the commentents in aquamacs dual card thread, and noticed he said NEVER to delete the nvidia kexts. So I restored the kexts from a recovery install, and used the EFI for dual 9800GTX (because of similar properties). I rebooted, AND IT WORKED. 2 Cards detected, full hardware acceleration, 4 monitors, everything.
The problem is, I rebooted after that, and it stopped working again. I am stumped... It worked fine, I change nothing, I reboot, and it's broken again. It now hangs JUST before it's surposed to go into graphics mode to show the login screen. No panics, no errors, it just stops...
I have no clue... but now I know my board/cards are at least capable of doing this.
TLDR; there are no good working DSDT's to be found for this motherboard, but with the right kexts you don't need one, and dual video cards only work without any of the crappy DSDT's that claim to be for this board.
Sorry for the spam, but I'm hoping this might be of some help to someone somewhere.