For Exadata Х6-2 there were 6 possible configurations of RAM at database nodes:
However, for X7 there are some changes:
Maximum memory of 1.5 TB not supported in Eighth Rack (ER) due to single processor limitation
In X7-2 user can have only 3 levels: the 384g, 768g and 1536 g now for RAM, a new memory stepping for X7-2.
As it was for Х6-2 family if you want to extend from say 768g to 1T you need to remove (and throw out?) 32g DIMMs and buy 1T of 64g DIMMs, that was very unpleasant for customers.
If you open the new price list then you can see some changes - you can buy X7-2 with 384 or 768 RAM from the factory.
X7-2-384 have 12 x 32g DIMMs ,
X7-2-768 have 12 x 64g DIMMs,
12 x 64 = 768g
So, if you buy X7-2 with 384g RAM and you plan to extend your memory in the future then you'll have to remove installed 32g DIMMs and install 64g DIMMs at place of 32g DIMMs .
Think if you really want this ?
My recommendation: take X7-2-768, it little more expensive but can save your money in the future.
Because in X7-2-768 model you have installed 64g DIMMs and it is possible add extra 768g without pain.