To clarify, when you SLI cards everything is "downgraded" to the lowest cards settings, this includes memory "amount", and clock speeds. This is becuase for SLI, everything must be equal between the two cards.
To furter explain, when you have cards in SLI, each card must share the same textures at the same time. This is why when you SLI a 1GB card with a 1GB card you effectivly only have 1GB of memory, not 2GB, becuase each card must share the same data that is stored in the memory, in essense it is duplicated. It is becuase of this, that that when you combine a 1GB card with a 512MB card, each card can only hold 512MB of data, and thus, effectlivly you only have 512MB of RAM.
If I got to techinical on you, sorry.