Look at your screenshot.
Your laptop uses
945PM with 32bit's
PAE. So it can't address memory above 4GB.
In your laptop memory above 0x80000000 address is used by internal devices. So 2048 Mb are available.
PCI bus use I/O addresses: from 0x80000000 to 0xFFFFFFFF. Because of it the chipset can't use addresses higher then 0x80000000 for operating memory.
Theoretically this range can be reduces to 0x98000000-0xFFFFFFFF. It should increase maximum memory to 2432 Mb.
In the lucky case
PCI Express Port 3 can be disabled, so
PCI bus's range can be reduces to 0xB8000000-0xFFFFFFFF. It should increase maximum memory to 2944 Mb.
Make new screenshot of Device Manager: click View at the top of the Device Manager window, click Device by connection, then select bus PCI and then expand Intel® N10/ICH7 Family PCI Root Port - 27D4.
I think it's used for
ExpressCard slot and can be safely disabled.
Update: maximum memory limits were decreased because of
TOLUD feature: [2:0] bits are reserved.
What kind of DIMMs do you have for this laptop?