I would also depend i would think on how much ram you have now and how many slots your mobo has.It may also be that crucial are covering there own back,as 2 pieces of 256 ram being faulty is less likely than 1 512 stick.
I read what they said, they are talking about if your computer supports dual channel memory. If it doesn't you're as well getting the 512mb as one chip. To post the whole thing "Many DDR and all DDR2 desktops and motherboards have a built-in feature called dual channel that allows you to utilize the bandwidth of two modules at the same time. For these systems, you need to install modules in pairs for maximum performance, so if you're going to be adding 512MB of memory, you are better off buying two 256MB modules, as opposed to one 512MB one". This is for motherboards with dual channel only.
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