You're almost certainly running a 32-bit version of 10.04. 32-bit OS's
can only address 4GB RAM (a lump of which you always lose to hardware
and OS ... and graphics if it's on-board shared memory, such as in a
laptop).

IIRC, W7 is all 64-bit now, hence being able to see the larger amount.

In order to benefit from the >4Gb RAM limit, you'll need to run a
64-bit version of the 10.04.

Or rebuild a kernel with the CONFIG_HIGHMEM64G option enabled. Your 32-bit
system will then be able to work with much more than 4GB of RAM.

Lucid's default kernel is built with CONFIG_HIGHMEM4G enabled.

Didn't want to complicate matters by getting into kernel rebuilds ... lol.

Are you sure it's enabled by default in Lucid? Robert stated he was
running 10.04 (32-bit), but was only seeing 3Gb of his 6Gb.

It's CONFIG_HIGHMEM *4* GB (not 64GB) that's enabled, hence only 3GB+
being visible.

Yes, I installed the 32-bit version of 10.04. I did not know about the
RAM issue. May try the 64-bit, but had heard it was not as stable as the
32-bit.

Where in the world did you hear that it's not as stable? 64 bit ubuntu
come highly recommended on servers. I've got several out there and they
are rock solid. What you probably heard is that the desktop application
support for 64 bit linux is not as good as it is for 32 bit linux.

There are several reasons regarding the 3GB detection. The most prevalent is
that either the MotherBoard/BIOS combination needs updating and can be run with
a PAE enabled kernel, and the BIOS has not been updated nor the kernel, or the
MB/BIOS is one that does not support memory re-mapping/hoisting which is now
usually a feature in the newer MBs/BIOSes.

There is a thread at Ubuntu Forums entitled: PAE kernel does not see all 4GB of
RAM to which sanderj
[url=ubuntuforums.org/showthread.php?t=1399216&page=3]replied in post
#21[/url] and listed a python script at:
http://wattcher.015.info/check-my-hardware.py which will tell you what memory is
usable and some hints about what to do.

I recommend you download or copy/paste it and run it (as root req'd):
# python check-my-hardware.py