The Windows 7 SP1 comes with all the previous security, performance and stability updates, as well as some improvements to features and services. Microsoft advises users to use the regular Windows update system, although manual download will probably be the faster way to go.
Depending on your method of choice, you will need anywhere from 750MB to 7,400MB for installation. Naturally, any pre-release versions of SP1 will have to be uninstalled.
You can find the SP1 here.