2k (I think that's what you mean and not 2x) will work with older motherboards, 4k will not (that is my understanding).
But I don't see any reason why they would be seen as 16MB under 2k and 64MB under 4k.
The 2k/4k is just the rate of refresh of the cells.
[link|http://www.oempcworld.com/generic.html?pid=16|http://www.oempcwor....html?pid=16]
The only thing I've heard of with 4k in older motherboards is that you will lose data and may damage the motherboard/RAM.