Hi, I want to know how would you manage memory and a backup system?

I plan to make a website. I want to know how I can make sure I always have memory available and avoid a crash. The website is going to be like myspace.

So I would like to know if their are many users or as the website gets popular how do you deal with adding more and more hard drives for memory and how do you do backups.

I mean how do you backup a system and not allow backing up virus or bad cruppted files?

thanks for your time.