Enriguecano, if you view the available amount of ram via Control Panel\System on a 32 bit XP installation, it only shows the amount of ram that XP can use. (Example: Your 3 GB available versus 4 GB installed.)
With 32 bit Vista installations, this was changed by Microsoft to allow Control Panel\System to show the physical amount of ram installed. However, you still only get ~3 GB of the available 4 GB for use.
Now, the reason for your computer "losing" ~1 GB of usable ram is due to "addressing." In practice, 32 bit OSs can only allocate a total of "4 GB of system resources." What is included in this 4 GB? (System ram = ~3 GB, video card ram = ~ 512 MB, and another rough 256-512 MB "lost" due to allocating other parts of the system board.
(Example: On my 32 bit XP Pro installation with 2 256 MB video cards in SLI, I would lose 256 MB if only one card was installed. However, if I installed the 2nd card in SLI, I would lose an additional 512 MB. This was due to the system having to lose an extra 256 MB in order to even communicate with the 2nd card. Hence, addressing.)
Now, with all the above said, there are pluses and minuses to upgrading a 32 bit OS to a 64 bit one. I, despite my system having 4 GB of physical system ram, am in the camp that says that keeping 32 bit XP Pro on my system is better than upgrading... even though I have the software (Win 7 64 bit Home Premium retail) available. This is due to the following reasons:
1. I have no need of DX 10+ on my system. (I don't play the newest games at this time.)
2. Due to my acknowledgment years ago of the video card SLI issue, I upgraded from 2 256 MB cards to 1 512 MB card... hence recovering 256 MB of usable ram and getting a better video processor as well. (2 8600 GTs to 1 9800 GT.)
3. The copy of 32 bit XP Pro on my desktop is an OEM copy. (I upgrade it, I lose it.)
4. The Nvidia motherboard in the desktop is beginning to die anyway. (No use to introduce another potential issue when I know how the system runs with the current OS.)
5. The desktop isn't used for Photoshop, CAD, etc. (It is basically a web browsing and older game playing machine.) As such, keeping it XP Pro actually helps. (Since, as many of us know, new OSs always make older games not want to run.) [Note: I keep a Windows 98 machine around for this same purpose... to run games that won't run in Win XP.]
6. Despite my system having 4 x 1 GB of physial ram, with only 3 GB available, you don't lose "all" of the gain. The system still uses the chips in duel channel mode. I just lose 1 GB of maximum available.
Again, there are many reasons to upgrade... my desktop just isn't one of them. (My mate's pc, on the other hand, could use the upgrade due to the 20+ browser tabs open at all times, etc. But, even then, the other benefits are lost.)
To each their own.