Tom, Are you using many custom materials? That relatively low video RAM caught my eye. I used to have Radeon cards. Going from 128 MB to 512 MB caused a noticeable improvement. Not earth-shattering by any degree, but definitely noticeable: movement was spiffier, there were less dropping into wire-frame and loss of a few images when orbiting.
Your custom materials reportedly rely more on videocard memory than system RAM.
Back to general maintenance, a nice little free utility program is called CCleaner - C stands for crap, so I naturally like it. It just deletes accumulating crap, like system logs, cookies, assorted temp files, etc, from one convenient location. And under Option/Cookies it allows you to manage what cookies you would like to always keep around (like the cookie from this forum).