You can
place there browser cache and temporary files, that does speed up the browser.
But yeah, get a SSD. Even a crappy one is better than the best HDD as far as response times go.
More ram is useful if you want to play with virtual machines though. That is, if you want to have Windows 7, Mac Osx, and Linux Ubuntu going at the same time.
Along side that, as my memory on this PC seems to go to 0 free sometimes. I assume that this will increase the amount of Page Filing too.
The number you should look at is the "Available".
Win7 does preload as much as possible into RAM, trying to guess what you will need more, to improve responsiveness.
In case more RAM is needed by something else, some of that preloaded stuff is deleted to make space.
So, even if you have 0 Free you can have more than 4 gigs Available.
All my other PC's i built have never said a usable amount. Is that something wrong?
The short version of the article above:
It is likely because that PC has a 32-bit version of Windows (Control Panel --> System to see if it is 32 bit or 64 bit). It cannot see more than 3.2-3.9 GB of RAM.
It's just a software limit (in the more distant past it was also a hardware limit), all ways to sidestep that without using a 64 bit windows version tend to suck.
Technically even 64-bit version does have a maximum amount of usable RAM, but it's a ridiculously huge number.