As computers memory capacity grows and grows, modern OSs have to be updated to address it. The 32-bit (x86) version of Windows Vista pre-SP1 (Service Pack 1) would not properly report memory over 3GB. With the new SP1 update, it corrects this problem.
Here is an excerpt from a Microsoft KB (929605) article: "If a computer has 4 gigabytes (GB) of random-access memory (RAM) installed, the system memory that is reported in the System Information dialog box in Windows Vista is less than you expect.
For example, the System Information dialog box may report 3,120 megabytes (MB) of system memory on a computer that has 4 GB of memory installed (4,096 MB)."