Tell us a little more about your system. How many and what kind of video cards are you running? How many monitors?
Right now I'm running a big ol video card (OCed GeForce 6800) and three Matrox G450s on 7 monitors with a crapload of charts. My CPU (it's noon, so it's slow) is running at about 20% (3 ghz). My RAM is underused (1 gig).
You can tell if your RAM is getting hammered because Windows (assumption here) will cache out to disk. So you'll see your hard drive light on a lot. I wouldn't recommened less than 1 gig for new systems, but 512 meg seems reasonable.
To check if your CPU is pegged, pull up the task manager and look at the performance tab.