ink, standard versions of windows xp will only recognize up to 3gb of total system memory (including the video card)...without tweaking anything.<snip>
No, that is not quite right. Windows XP does recognize all 4GB of system RAM, but the 32 bit version of the OS can only *directly* address 4GB of RAM. In order for all the other RAM in the computer (video card, sound buffers, any other hardware buffers, system BIOS and other BIOS's...) they have to be mapped in the 4GB address space, thus reducing the physical amount of system RAM as all the other RAM overlaps that space.
You can directly subtract the amount of video RAM your video card has from the 4GB limit and this gives you an idea of what the absolute amount of system RAM the CPU will be able to use for programs/data. All the other bits and pieces in the computer will normally take away about 300MB (+/- 128MB) of RAM.
The reason Windows 7 32 bit will allow all 4GB of RAM to be used is due to Microsoft finally using (the standard is over 10 years old, for crying out loud) the Intel PAE (physcal address extension) instructions to allow a 32 bit kernal to physically address up to 64GB of RAM. However, the access speed to the addresses above 4GB is much slower than the addresses below 4GB.