SolidWorks actually uses multi-thread technology for certain functions. This is different from when we talk about multi-cores. Multi-core describes the CPU. It is the operating system that ultimately decides what Core each Thread runs on. Windows 7 is said to utilize multiple processors more efficiently compared to Vista.
Only certain functions of SolidWorks can actually utilize the multi-thread technology. Running simulation analysis, rendering images, file opening and user interface activities (redraw, dialog boxes) can take advantage of multiple cores as there are multiple calculations that can be determined simultaneously. When opening documents in SW2010, a view-only state is provided where you can zoom, rotate, etc while the actual file and its components are opened in the background. Once the file is fully opened, the environment is changed to the normal editing state. In contrast, the solving process used for parametric modeling is by nature linear and can only be run on a single thread. Consequently this cannot be spread across multiple processors and one core will take the brunt of the calculations.
In general, rebuild times depend on the speed of the CPU, simulation analysis and image rendering depends on having more cores and RAM, and file opening mainly depends on the hard drive speed.
Keep in mind that the fastest machine is no substitute for good design practices. Minimizing in-context references and utilizing lightweight modes will reduce the load on the processor.
For more information on SolidWorks and multi-thread, check out another blog post at: http://www.javelin-tech.com/blog/2011/01/solidworks-multi-threading/