You could also check which memory slots are in use. If your system is capable of using dual channel memory, have you got the memory inserted into the correct slots in order to take advantage of this (refer to your motherboard manual)? As regards installing more RAM, sure you can do this but it might have the opposite effect and actually bring your score DOWN. This is because some boards have an issue with having all memory slots filled and having the memory synchronised due to timing issues. This is usually resolved by the system reducing the memory bus speed so that the limitations caused by the differing electrical track lengths between the memory slots and the memory controller are minimised. For example, my system uses PC3200 (DDR1-400) running at 200 MHz. If I was to install more RAM of the same specification, then it would not run at 200MHz - rather it would run at 166MHz, the equivalent of PC2700 (DDR1-333). This is a reduction of 17%.
I also notice that you are running 32-bit Vista. As such, it is only capable of addressing a total of 4GB of memory space (including addresses reserved for hardware and I/O devices), so the physical memory available to such a system is usually somewhere between 3GB and 3.5GB dependant on the system hardware. In order to be able to take advantage of 4GB and above, you need to be running a 64-bit version of Vista. In short, don't waste your money on more RAM if you cannot access it due to address limitations. Even if the extra memory cannot be accessed, then simply having it installed can cause the memory that you can access to run at a lower speed for the reason described above.
Dwarf