The 6GB 780 is a bit pointless IMO. Reviews show no useful performance gain vs. 3GB 780s,
and for those situations where one can really strain the VRAM (very high-res/detail @ 4K),
the raw power of a 780 just isn't enough anyway; indeed, even two might be a stretch -
check 4K reviews of 780 Ti SLI, the gain of 780 Ti SLI over 780 SLI at 4K is often enough
to make all the difference between smooth and a bit choppy, though in some cases the
gap can be surprisingly narrow (very much depends on the game).
Thus, if you're playing at 2560x1440 or less, you really don't need more than 3 or 4GB,
in which case you're better off with a good 780 Ti, which is quite a bit quicker than a
780 (and as so often happens, oc'd 780 Tis are now cheaper than reference cards),
though at standard 1920x1080 even a single 780 should give good frame rates for
most games. Personally I'd get a 780 Ti if I could, fit a 2nd one later if I wanted some
extra speed (the release of Maxwell should force 780 Ti prices down, suddenly
making SLI setups more affordable for those who only have a single card).
As to whether you have enough for a 780 Ti, I don't know - USD/UKP rates are
going a bit whacko atm, so it's harder to judge these things than it was a few
months ago, but have a look around, might be some deals to be had.
To be fair btw, AMD's 290X *is* fast, and cards with proper cooling are good, but I
still don't like AMD's drivers (benchmarked my first 7970 last week, some peculiar
oddities when running tests, eg. DX error after failing to lock a fullscreen mode,
that sort of thing). They do have a natural VRAM advantage (4GB instead of
3GB), but again this only shows up in very high-res/detail scenarios.
Atm the only thing sillier than buying a 6GB 780 is buying an 8GB 290X or whatever...
So what resolution is your monitor? Any plans to change it in the near future?
If you're gaming at 2560x1440 or less, get a single 3GB 780 or 780 Ti (the latter if
affordable), add a 2nd later after Maxwell is released for more speed if needed,
assuming your mbd supports SLI of course.
Ian.
PS. Watchout for reviews which don't use systems that have enough CPU power to drive
such powerful cards, these reviews end up introducing CPU bottlenecks that shows card
configs closer together than they would otherwise be if used in a better system.