|If you have a 32 bit version of Vista, your system may run noticably better with 3gb instead of 4gb.|
32 bit XP and Vista have a 4gb virtual (memory) address limit - for both what the ram needs and what Windows itself needs.
You end up with a situation where only a bit more or a bit LESS than 3gb of the ram is actually being used, depending on the system, when you install 4gb.
e.g. On a friend's system with an Asus M3A78-EMH HDMI mboard, XP Home, and a Sapphire 4850 PCI-E X16 video card, I tried both 3gb and 4gb in it - it has MORE available memory in System Information when 3gb is installed, and Windows runs very noticably better with 3gb.
You probably are not using any program that needs 4gb in any case.
Ultimate Memory Guide
How Much Memory Do You Need? etc.
Lots of things can cause Windows to slow down. Your comparison is only valid if absolutely nothing else was changed.