High density will probably refer to the fact that the RAM has 512MB per side on the stick.

SDRAM is Synchronous Dynamic RAM - AFAIK, all the RAM that fits into a standard desktop PC is SDRAM. Mine certainly is.

As far as the compatibility goes, if it says it works with the Via KT400 chipset, it will work with that chipset regardless of board manufacturer. I think that pretty much any DDR333 (PC2700) RAM will work with the board.