1 gig chip goes bad you got nada. 2x 512 and still would have a 512
chip functioning.
but to get too 2 gigs then you would need to ditch the 512's possibly
and get 2 new chip's.
You just price them. I think usually there is a size cut off when
the larger chip costs much more than twice the smaller chip. The
benefit of one larger chip is upgradability, you can fit twice the
memory into the same mainboard (assuming the board supports it).