I doubt that you will find ANY SDRAM for sale that is not High Density.
The eMac will not provide any advantage (or disadvantage, either) with an identical pair, dual channel is not supported. You will simply have two chips with the same capacity.
It will work just fine with 1 PC2700, and 1 PC3200. The memory bus supports either, but PC3200 does not operate faster in the eMac, so no speed advantage.
I prefer PC3200 because it is more common, easier to find, and often sells for lower price.