CPU Usage Percentages
Since you have four cores at your disposal, maximum CPU usage would be 400%, or 100% of each core presuming that each application is intensive enough to make full use of each core. This isn’t often the case however.
Using Photoshop as an example again, it may maximize one core at 95%, or thereabouts, while saving or editing a high-definition image. It is unlikely to spread power to more than two cores in any case. This would be similar for other image editing programs.
Running instances of multiple high-quality videos will likely use less than 25% of each core. However in the case of using editing or capturing programs, such as Windows Media Encoder or an intensive program like Panorama Factory, you will likely take advantage of all four cores and it should result in relatively high percentages. Rendering and encoding applications are mostly the ones that are optimized for a quad and will likely use full resources.
Most performance-intensive games (such as Quake 4, Elder Scrolls: Oblivion, Rainbow 6) will only use two-cores effectively. Something like the new Elder Scrolls game is likely to use 80% and 20% of two cores, while utilizing very little of the others.