Apple is recommending installing on a separate partition from FCP 7/Motion 4 but frankly I've been running FCP X/Motion 5 on the same boot drive with no issues, as have many others. Installing FCP X and the other apps does NOT overwrite your existing FCS
You need an OpenCl-capable graphics card or Intel HD Graphics 3000 or later with at least 256MB of VRAM. The link includes a chart of cards that do NOT meet the spec - of note is that the ATI Radeon X1900 XT, which used to be THE card for Motion (before
You can get the user manuals for FCP X, Motion 5, and Compressor 4 from the Help menu in each application or online - you can also print them to PDFs for easy offline access - I put my on my iPad to make them easy to grab.