What type of programs are you intending on running? If you only need to run Office, Email, and Web applications then 4GB of RAM should be enough, but if you plan on doing any creative professional work, virtualization, or scientific computing then 8GB of RAM or more would be preferred.
As for Windows, Apple provides Boot Camp, which will allow you to install Windows to a second partition on your hard drive (dual-booting). However, this requires you to shut down the Mac side in order to run Windows, and vice versa. An alternative to this is to use a virtalization program like Parallels Desktop or VMWare Fusion, which will allow you to run Windows within OS X. The benefit is you can run programs side-by-side with your Mac programs, but at a cost of overall performance (and dong such will use more system resources and thereby require more RAM, so 8GB would be a minimum I would recommend).
The only options for building a supported Mac system are through Apple's online store, where you will configure a Mac system for your needs by choosing items like CPU speeds, RAM, and hard drive options. Apple does not support Blu Ray on its systems.