PCI devices always "steal" RAM, even on a system with less memory. You just don't notice because PCI devices are usually sitting at an address beyond the end of physical memory.
Remember, 32-bit processors have a 4GB address space (from 0x00000000 to 0xFFFFFFFF). If you have 1GB of RAM, any address above 0x40000000 is free.
Most PCI devices are memory mapped. This means they are mapped to a memory address. If the CPU wants to talk to your Ethernet Card, it reads and write to a certain memory address, which happens to be where your Ethernet Card is listening.
The memory address that your device uses is determined by the BIOS at boot time (although the operating system can remap the device later). The device can claim anywhere from 0 to 6 memory regions, and they can be just about any size.
Exactly how many memory regions, and their size can vary gratly by the device. Ethernet cards tend to use very little. Graphics cards, however, can use a lot. Grpahics cards have a lot of onboard RAM, and that RAM can be often addressed by the PCI bus. If you have a video card with 128MB of RAM, it may claim a 128MB window on the PCI bus. Some high-end SCSI controllers claim a lot of address space as well.
If you want to see the addresses claimed on your system, go to Control Panel - System - Hardware - Device Manager. Click View - Resources by Type. Expand the memory selection. You should see all the memory-mapped devices in your system, and what they claim.
Since you have a 4GB limit, your system needs to make all of the devices, and your RAM fit. That's why you see less RAM. If it didn't allocate enough address space for your hardware, the device wouldn't work (which would be really bad). So, it does the safe thing, and give you less ram.
You can do one of the following:
- Live with the limit. Maybe sell the spare 1GB module.
- Get a CPU with PAE. PAE give you a 36-bit address space. This lets you use RAM at addresses above 0xFFFFFFFF. You will need to use the "/PAE" switch during bootup.
- Switch to a 64-bit CPU. You'll need a 64-bit operating system.
One little tidbit. Most PCI devices are 32-bit, so they have to live at an address below 0xFFFFFFFF. There are also 64-bit PCI devices that can be mapped to addresses above 4GB, but they're another issue entirely.
