Intel Chipset 945PM – Running 4Gb Ram?

One of my biggest pet peeves with the Aspire 5560 so far is the lack of a 64-bit processor option. Thankfully this looks like it's about to change, but I'm stuck with the 32-bit Core Duo, hence my problems.
I ordered mine with 2 x 1GB sticks of RAM, but I use Photoshop and Matlab with very large datasets, so I decided to try and upgrade it to 3-4GB (even though i know that each app can only use 2GB in a 32-bit environment). I have 8GB on my dual Opteron desktop running WinXP 64-bit, so I realize some of the issues with >3GB RAM.
Anyhow, to report my findings:

1) I first upgraded to 3GB RAM with 1x 2GB module and 1 x 1GB module. Windows saw 2.99GB (close enough). However, some people have mentioned that the Intel 945 chipset uses dual-channel RAM if the two modules installed are equal in size, so theoretically one would see a drop in bandwidth (5%?) if I had mismatched modules like this case.

More worrying, 3D performance would drop a lot more since video RAM is shared main RAM, and losing dual-channel bandwidth apparently impacts video performance more (do a Google search for this issue on MacBooks if you are interested, there was a lot of discussion about it).

Anyhow, when I tested memory bandwidth using Sandra, I indeed found a small drop in performance ranging between no change to about 2% or so slower. 2D and 3D performance dropped appreciably in certain tests using PerformanceTest 6.1, but stayed the same in others. The Simple 3D test showed a almost 18% drop when I moved from 2GB to 3GB. This is disturbing.

2) To get around this I managed to borrow another 2GB module and install it. Now I have a total of 4GB of ram, 2 x 2GB. All modules are 667Mhz (PC2-5300), so none of the speed issues are due to underclocked memory. This was also checked in Sandra which showed 667Mhz RAM.

However, Windows now only still sees 3GB!! I was expecting 3.5GB of RAM, due to the mapping of BIOS and video addresses into the top part of the 4GB address space, a well known issue in XP 32-bit. Pressing F1 to go into BIOS shows 4096MB of RAM installed, ok. however, if you turn on the full memory check option, upon startup it does the memory test which only goes to 3GB of extended RAM.

What gives? I'm feeling cheated out of my 0.5GB of RAM that I _should_ be able to see in Windows but am not. However, the ironic things is that memory bandwidth and 2D/3D performance are now back to the same levels as with the 2x 1GB modules, as dual-channel is now restored!

Anyone have any ideas how to get the remaning 0.5 or 1GB visible in Windows? There is no memory remapping option in the X60t BIOS, and PAE is on by default since DEP is on.

Thanks a lot, hope this helps others trying to upgrade to 3 or 4GB RAM.