Also (and I'm not sure why, so if anyone can fill me in on the details, I'd appreciate it) high-density RAM works best on newer systems that don't have specific DIMM size limitations. (You know, the kind that you could theoretically toss a 1GB DIMM into.) Low-density RAM however works in those ancient machines where they had significant memory limitations. Some of those old machines can't even take more than 128MB into each DIMM slot.
Again, I don't know why the density matters. I just know that I've seen it a lot of various memory reseller web pages. If anyone knows just exactly why density matters, I'd like to know.