Starting with Vista, MS changed the way it uses memory. Unlike XP and earlier, it tries to maximize performance by making greater use of RAM. The fact that 80% is used at any one time is not indicative of anything. Windows keeps a lot of things in RAM in attempting to prevent having to reload it from the hard drive. This makes a great deal of sense since RAM is roughly 100,000 times faster than the hard drive.
Since you're running Vista, you're likely using the 32 bit version. In that case adding more RAM would make absolutely no difference unless you want to upgrade to a 64 bit version.