The first reason why it is rebooting is because of a kernal fault, the reason it keeps rebooting is a setting at: "Control Panel / System / Advanced tab / Startup and Recovery / Automatically...
x64 allows for addressing more than 4GB RAM. Also the x64 version of Windows 7 has some additional security so I would get that version anyway. Make sure you buy an SSD drive for your C: and you'll...