If you want to max out your computer to 4GB, but only 3GB shows up, did you get ripped 
off? Not necessarily. There are 2 types of operating systems, x32 and x64; otherwise knows as 32 bit and 64 bit. If your computer doesn’t specify which one, then it is x32. Only when it specifically reads x64 near the Windows sticker is when you know it is x64. You cannot buy either one, your computer’s architecture is inherently either x32 or x64. So, you inherit these types.
Now, that we’ve covered the bits, let’s talk about the bytes. Here’s the graph the each type can handle:
- Windows XP: 32bit: up to 4GB; 64bit: up to 128GB
- Windows Vista: 32bit: up to 4GB, except 1GB for Vista Starter; 64 bit: up to 128GB
- -exceptions Vista 64-bit: Home Premium 16GB; Home Starter: 8GB
Now, that we’ve gotten the limitations out of the way, we can talk about what to do if your RAM only shows up with a total of 3GB when you’ve installed 4GB. First of all, your computer needs to allocate this RAM to your video adapter and overall hardware. Then, the rest of the RAM is allocated to applications and the operating system. So, although you’re not getting all of your 4GB of RAM to your software and Windows, the rest of the RAM is being used well.
The hacker fix: If your computer doesn’t seem to recognize 4GB, there may be a trick you can do. Open your boot.ini files and remove the Read-Only and Hidden attributes. And then delete the 3GB switch. Then, add the /PAE switch and reboot your computer.
Keep in mind that the hacker fix maxes out your RAM expectations above Windows expectations. So, your results may be short-lived and so may be your computer’s life. So, it’s recommended to just leave your computer alone and let Windows be the judge of what memory, whether 3GB or 3.5G, may be the total amount allocated to the operating system and applications.