I just checked your system spec,
Although Microsoft say 32-bit versions of Windows Vista support 4GB of RAM they actually support 4GB of *address space*, not RAM.
The useable amount for 32bit is around 3.2-3.4gb but windows will see it as 3.
You need a 64bit operating system to run more than that.
7 premium 64bit will allow you to use 16gb and Ultimate will allow upto 96gb iirc.