Strange display problems: lines, missing icons, pieces of windows that stay on screen...

Question

My white 24" iMac has recently started having problems with the display. Because I'm able to capture it with Grab and due to the nature of the problem, it doens't appear to be hardware related. But, not sure. See the screen cap I took below as an example of one of the problems I'm seeing.
I'll try to describe a few other display anomalies I'm having:
Moving a window leaves the background that it was on intact all around the edge of the window, where the drop shadow would be.
The Dock, which is set to be hidden until my cursor moves into that area, will appear sometimes with some of the icons missing, but, the shadows for those icons are visible on the Dock. Or, there are weird lines and patterns around each icon.
While using Safari and a few other apps, I'll frequently get colored lines and patterns that suddenly appear across the screen, and then disappear when my cursor moves over that part of the screen.
When I click on the minimize button, the window uses the Genie effect to move into the Dock, but, often leaves a bit of the window on the screen. It will sometimes clear when I drag another window over the area, or the window I'm dragging will start looking like it does in the screen cap below.
My graphics card is a NVIDIA GeForce 7300 GT. I updated the firmware to ROM Revision 3467, but, it didn't seem to make any difference. The only think I can recall doing before this problem happened is I installed the iTunes 10.4.1 update. I'm running OS X 10.6.8 (Snow Leopard).
This iMac is about 4.5 years old, so, it's not covered by warranty anymore which means I can't take it to an Apple Store for troubleshooting. I'm hoping someone knows how I can fix this. It just started doing this a week ago. Thanks!
Tom

Answer 1

Hi Michelefree,
My iMac does not have a separate video card, so, I was not able to replace it as suggested above. Apparently with the Intel iMacs, or at least the one I have, the video "card" is actually a few chips on the logic board. I was told the only way to fix this is to replace the entire logic board, which is too costly (no longer within warranty).
I did a bit of research and discovered that the problem seems to be related to how hot the computer gets. I noticed this, too. When I started using my iMac first thing in the day, no problems. As the day progressed and the computer heated up, I'd start having all these video problems.
I found a utility called smcFanControl which allows you to speed up the mininim fan speeds inside the computer, which helps keep it cooler. Since I installed this, I've had very few video problems. So, it appears to have "solved" my problem for now.
I wish there was a better solution than this band-aid fix, but, it's a lot cheaper than having the logic board replaced or buying a new iMac, the only choices I was given by Apple. Hope it works for you, too!
Tom