As promised, I have looked over some old notes and emails.... They seem to confirm what i thought, although it is equally possible that I'm all confused and have been from the outset.
The issue arose when I first built my PC. I had a 256MB GC to begin with. With that, Windows showed the available RAM as 3.24GB (I had 4GB installed). I then swapped this card for a 512MB GC. Windows subsequently showed 2.99GB as available.
I spoke to someone who was guiding me through the build at the time. He had used the same board in several of his builds and likewise thought that the DP35DP could re-map memory. So he was surprised by this as well. I also emailed Intel at the time for their opinion. They confirmed that the DP35DP does not re-map memory for IO devices etc.
Here's an explanation (I have more, but think this is the clearest) of what I'm meaning here by "Remapping" (just to ensure we are talking about the same thing):
.......many motherboards have a BIOS feature named 'memory remapping' (aka 'memory hoisting'). Without getting into too many complicated explanations, essentially this is a clever workaround that makes it possible to map some of the missing RAM segment higher up beyond 4GB, where it will not conflict with any I/O device address, so its contribution to system RAM is returned to the pool (if you want a more in-depth technical discussion of the details behind memory remapping and 4GB support, Asus provide a handy PDF guide on their web site (http://dlsvr01.asus.com/pub/ASUS/mb/4GB_Rev1.pdf
Some motherboards provide memory remapping as a BIOS option, and enabling it may suddenly let Windows see significantly more of your 4GB installed RAM. Others implement it automatically. However, some cheaper motherboards primarily intended for consumer use don't feature remapping at all, on the grounds that few consumers are likely to need more than 3GB of RAM, in which case you may see your available RAM drop each time you install extra hardware, and you won't be able to claw back any of your missing memory at all........
washu and anyone else, I'd be really interested to hear your thoughts on the topic, both with regards to whether this is correct, and also with how it might impact my choice of graphics card when it comes to choosing amount of memory.