To answer your first question as to whether or not you should get the 2GB cards, the answer is yes.
When you put two cards in SLI/Crossfire the contents of the video memory on each one will be more or less identical. If the cards are rendering alternate frames, it stands to reason that the content in the frames will be more or less identical and subsequently the geometry and texture information needed to render it will also be the same.
The outcome of this axiom is that putting two cards in SLI/CrossfireX does not double your video memory and consequently a performance hit caused by limited video memory will not be alleviated by adding a second card. Many games are starting to push past the 1 GB barrier, Crysis2 actually requires a card with 1GB when running the HD texture pack and even then it can be a slug.
Extra video memory is also necessary to serve the primary purposes of using two cards, displaying at a higher resolution, at greater detail and with better picture quality. I haven't measured it myself but I play BF3 at 2560x1600 and I imagine that I'm pushing well over 1GB on my HD6970.
So yes, I would say that it is a good idea to invest the extra money in buying the 2GB cards. Were I you I would actually buy 2 HD6970s (or one, and add a second later if you so desire), the performance difference between the HD 6950 and the HD 6970 is pretty noticeable.
To answer your second question: 750 watts is cutting it close but it should suffice. As long as you're not running 4 Velociraptors and overclocking your CPU it should be okay.