Windows Vista SP1 focused on a variety of aspects including the way the operating system deals with the underlying hardware resources, thus bring increased performance. Following the introduction of the service pack, the operating system will report more RAM than it did as in the RTM build, in various scenarios.
The first example comes from Michael S. Kaplan, Technical Lead on the International Fundamentals team at Microsoft, and is the confirmation of what Microsoft promised that Vista SP1 will deliver to 32-bit machines with 4 GB RAM. Before Vista SP1, previous x86 Windows platforms, including Vista RTM, reported only a portion of 4 GB RAM, keeping in line with the limitations of the 32-bit operating systems to address system memory beyond 3.3/3.5 GB. SP1 changed this for some Vista RTM machines powered by 32-bit processors.
“With SP1, Windows Vista will report the amount of system memory installed rather than just the amount of system memory available to the OS. Therefore 32-bit systems equipped with 4GB RAM will report all 4GB in many places throughout the OS, such as the System Control Panel. However, this behavior is dependent on having compatible BIOS, so not all users may notice this change,” reads an excerpt of the Notable Changes in Windows Vista SP1 document.