Question : Problem: XP did not recognize all memory

We want to upgrade some HP desktop pc's to 4gb RAM.

HP DC7100SFF
HP DC7600SFF

After we installed the 4GB ram, Windows XP recognize only 3GB.

In the BIOS we see 4GB

After some research, I found the following comment on the HP website:
Above 3 GB, all memory may not be available due to system resource requirements.

Does someone know what this means?
Is it still possible to use 4GB ram on these HP machines?

tnx

Answer : Problem: XP did not recognize all memory

First, to correct a couple of errors ...

"... Windows will only recognize 4gb total.... minus the page file and video ..."
"... You wont pick up the 4th gb of Ram as you will have a minimum 2mb Page file ..."

==> The page file does NOT occupy any memory !!   It's a file on disk that maps the virtual address space (2GB for each process unless you use the /3GB switch) to real memory.

"... Get a video card that's not onboard and you will gain some of that extra missing Gb " ==> No, this won't help.   If the system had less memory ... e.g. 2GB ... then it would make a difference;  but once you cross the threshold where the reserved addresses are limiting the available memory, it makes no difference.   The video addressing assignments in the upper part of the 4GB address space are there independent of whether there's a physical card with its own memory or an onboard video adapter.   If all of your memory could be addressed (e.g. you had 2GB of memory) then it would make a difference, because the memory assigned for the onboard adapter could be reclaimed if an add-on card with its own memory was used => but in this case it doesn't matter.
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As for your specific problem:   It's true that with a 32-bit OS you won't "see" all 4GB of your memory.   With 32 bits the total address space is 4GB.   But there are several elements which MUST be assigned ... and these are assigned from the top of the 4GB address space downwards.   After these are done, the physical memory in the system is mapped to the remaining addresses ... so you'll typically see something like 3 - 3.5GB, depending on the specific configuration of the system (sometimes even less than 3GB in systems with multiple video cards).

The reserved items in the upper end of the address space include BIOS shadowing, AGP aperture, I/O addresses, video shadowing, etc.

If you install a 64-bit operating system, the address space limits aren't an issue --> the reserved elements are still needed, but can be assigned well above the 4GB boundary, so you'll "see" all of your installed 4GB.