MasterOfTheMagicBox,
I know that more than a year has passed since you asked your question, so you probably have found an answer by now... but it is still marked as unanswered. At the very least, maybe I can help the next person who finds this thread through a search for the same info.
I don't have the model of laptop you do, but I do have an Asus F8Sp-X1 with the PM965 chipset. I can confirm that it does work with 8gb; it is running right now, with all 8gb recognized in the BIOS and in Windows. It also works just fine with Windows 7 x64 (and did so even before I upgraded to 8gb), which is what I am using. It would appear that the PM965 chipset, despite what Intel says, can handle 8gb without issue, even if they only "support" 4gb.
As for the failure of the Dell with 8gb to POST: Did the people who tried it use SoDIMMS with 16 chips each (8 per side)? The reason I ask is that I saw a comment on the Asus forum from someone who had also upgraded a F8S* to 8gb, and he had said it will only work if you use the double-sided modules with 8 chips per side (16 total per module). If that was what the people you read about were using, then it is probable that your assessment about the BIOS is right.
The modules I have are DDR2-800, and even though CPU-Z does report that the SPD correctly identifies the modules as PC2-6400 (DDR2-800), they are only running at DDR2-667 speed. The CPU in use is the T7800 (Core 2 Duo 2.6ghz, 800mhz FSB).
The PM965 chipset can use dual channel mode with two differently sized modules (which, in my AMI bios, is called "dual channel L shaped mode"). If you put in a 2gb and a 4gb module, the first 4gb will be dual channel; the last 2gb will be accessed in single channel mode. While this is a configuration that most of us would not prefer to use, it is somewhat common for laptop makers to configure their devices this way. My F8Sp came with 1x1gb and 1x2gb, for example.
When I was researching all this stuff myself in preparation for my memory upgrade, I remember reading an article about how MSI was showing a new laptop to one of the various tech bloggers out there, and he was surprised to see the two memory slots were populated with different-sized modules. MSI told him that in their tests, the single-channel vs. dual-channel difference was trivial. Later on, he tried some tests himself, and confirmed what he'd heard. It's not even close to the "running at half the speed" point that many people on various forums often will say about single-channel mode.
At the very least, 6gb in L shaped mode is almost certainly going to be better than 4gb in full dual channel mode.