3.3G RAM limit?

I have a fuzzy memory that there's some kind of limit that leads to
i386 topping out at three-point-something gigs of RAM.  Could someone
remind me of details?

What happened is....

I have two Dell "laptop" machines - they're pretty big for laptops, and
I think at least one of them was marketed as a "mobile workstation"
rather than a "laptop".  One of them is a Precision M6300.  That one is
amd64 and I've had it for a long time - but I stopped using it when the
video RAM developed problems.  (In text mode, something like 1/8 of the
character cells have certain bits forced on, displaying as a-acutes
when the screen is full of blanks and various other characters when
not.  In graphics mode, the screen has a regular pattern of dots,
presumably the same pattern in a different guise.  I found I couldn't
even run X unless I turned all acceleration off.)  This machine had 4G
of RAM, in the form of two 2G boards.

Recently, I got the other one, which is an Inspiron 9400.  That one is
i386.  It had 1G of RAM and worked much better, albeit of course
slower.  But the RAM looked identical, so I swapped the 2G parts into
the Inspiron.  And, hey presto!, the BIOS reports 4096M installed.

But it also reports only 3327M available.  The BIOS does steal some
RAM; when it had only 1G it showed something like 1024 installed, 1022
available.  4094M or 4093M or the like would not surprise me.  But
3327?

Of course, NetBSD/i386 reports about the same, 3326M available,
presumably believing what the BIOS tells it.

But of course 4G is a full moby of physical RAM for a 32-bit machine.
And 4096 - 3327 is 769, which is suspiciously close to 768.  And then
that fuzzy memory I mentioned above bubbled up.  So...is this physical
RAM limit to be expected, and is there anything to be done about it,
either theoretically or practically?  (Not that 3+G isn't plenty of
memory for the machine, of course; this is more in the nature of
wanting to understand what is going on and what is possible than in the
nature of a practical problem.)

/~\ The ASCII				  Mouse
\ / Ribbon Campaign
 X  Against HTML		[email protected]
/ \ Email!	     7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B