Ah, I was wrong indeed! My memory has failed me. It doesn't matter in this case, though, as that wasn't the point. I think. Definitely something confused me.
Anyway, I still guess it'd be hard to go for 320 MB. 512 MB might be easier, though more expensive as well. That's simply because the amounts seem to be powers of two, which 320 isn't. I think I've never seen a RAM module not be a power of two.