Your hardware configuration will depend on the size of your application. Sites expecting heavy traffic will probably need a clustered configuration where the load is distributed across more than one computer. However in the most common configuration all elements of the OpenCms installation (OpenCms application, database and front end web server) will reside on the one computer.
Single computer hosting all elements of OpenCms installation
Memory is the most important parameter when specifying your OpenCms hardware. No matter how fast your CPU, unless you have enough physical RAM, performance will suffer.
512MB is the recommended minimum (but you might get away with less if performance is not critical).
For CPU speed: the faster the better. But OpenCms is not exceptionally CPU demanding. If you are on a limited budget consider spending more on memory and less on CPU power.