what are your system specs i.e cpu hard disc etc. you say 1.6GHZ but is that a single core or a dual/quad core processor?
in the task manager theres an option to monitor performance in Vista and you can see how many hard faults you are getting in your memory. Don't worry about the term hardfaults( may be page faults but I'm working off my memory from 12 months ago) as these are the number of time Vista tries to access memory and it has been paged out into the swap file. So that it then has to reload the memory it needs from the swap file on hard disk in order to continue withthe program its running. This is a slow process compared to just a memory access and causes your computer to slow down drastically if there is excessive use of your swap file due to a lack of physical RAM in your computer.

Depending on your computer specs the amount of ram you can put in may be limited by the motherboard and your graphics card can also impact on the available RAm in a 32 bit windows computer as the memory addresses that graphics cards and other PCI/pci-e /isa are all mapped into windows from 4 GB down , if the required overhead of memory overlaps with your physical ram you won't be able to use the ram in the overlapped addresses. This is why if you put 4GB of ram into your computer , if you have a decent graphics card you may only have 3GB available to actually use.