memory for Intel's D845PEBT2 mobo

I built my first pc last year and want to add another stick of 512mb
memory. . . the D845PEBT2 has 2 rows, onw of which is already taken
with 1 512mb Corsair pc2700 DDR value select 333mz. . . .So when I
added the same type in the second slot. . .it says my total memory is
256mb. . .not the 1024 I thought I was going to get. . . I went to
Intel's website and could not understand the memory dispersal for the
D845PEBT2 mobo. . . can anyone tell me what I am doing wrong?. .
..thanks in advance. . . k