I'm using pacman 2.8.2, on pc with 192mb ram, 700mb swap and 333 mhz celeron. I found that, when tring to update more than 200+ packages at once, pacman eats all ram first, than eats all swap and after while, dies....
Well, it looks realy strange.
I will try to catch the problem (this problem is good reason to learn the pacman code) next few days.
you could submit a bug-report