User Me wrote:

But wouldn't I be "future proofing" my computer by getting the
overkill now which would be equal to more commonplace performance a
few years from now?

No matter how much money you throw at it, any top-of-the-line hardware
you buy now will be yesterday's news in a matter of months. If you
really don't care about money, there is no point in the question, just
buy the fastest and best money can get: two 7800GTX's.

If you care just a tiny bit about money, I'd try and get the best bang
for buck. You can pick up two 6800GT's for almost the same price as one
7800GTX. I know I did. To me there's little point in going for 7800's,
as you won't get double the framerate, and it certainly won't look twice
as good. With two 6800GT's (or Ultra's) you still get enough fps to last
you well into the next generation of games, and by then, you'll be en
route to buying two 8800GTX Ultra's.

If your point is in asking if you need 512MB in stead of 256MB per card:
for now, there isn't much performance increase. By the time you'll need
more memory on the graphics card, you'll be wanting a newer and faster
GPU to go with it.