i 'll show u daylight
the theory is that if (and only if your mother board supports the dual channel memory) u have two memory modules with same capacity the data is uploaded/downloaded form both ram chips at the same time (clock pulse) so effectively doubling the transfer speed than what u would have with one 512 chip.
in your case as u are going for 512 anyway ie either 2X 256 or 1X 512 so i think the first choice is better.
but again when u want to upgrade the ram u have to throw away the 2X 256 and get 2X 512 so u see. if u are going to upgrade recently the go for 1X 512.
so the plain advantage is improved speed.
well i guess similar topic has already been discussed so u can also refer to it .
i hope u can see daylight now