Vista 32bit conveniently fools users by displaying that it can use 4GB of installed RAM in the system properties, whereas it cant use use more than 3GB

to find it out, go to msinfo32
Total physical memory is the one OS actually uses
see for urself

My computer --> Properties


Msinfo32



Source: 4sysops - Why Windows Vista only sees 3GB memory in a PC with 4GB RAM and how Vista SP1 fools its users