Is it necessary to boot from USB?

[burgertime]

Just got a Pi. I installed RaspBMC and it randomly rebooted so that had to go. I found Xbian and installed using SD card but just after the first boot up, I was browsing videos in an addon called KidzPlace and Xbian dropped to TTY and said Xbian login:

weird... I've got a brand new Sandisk 16GB class 10 HC ultra 30MB.s SD card.

I feel like my experiences with Linux in the past (ubuntu and arch) have proven to be amazingly stable. I'm baffled why I'm having stability issues with this.

Anyhow... I wonder if it is necessary to employ a USB stick with my setup. If so, are there OFFICIAL step by step instructions on doing so? I saw the USB thread but honestly, at this point, I'm rather discombobulated and need something straight forward.

Thanks!