SHA256 is not going to be broken by Moore's law computational improvements in our lifetimes. If it's going to get broken, it'll be by some breakthrough cracking method. An attack that could so thoroughly vanquish SHA256 to bring it within computationally tractable range has a good chance of clobbering SHA512 too.
From Satoshi's post on BitcoinTalk: https://bitcointalk.org/index.php?topic=360.msg3520#msg3520