i have 2 gtx 780 tis in sli and they are insane. although, i really dont know if i needed 2. alot of games have graphics issues all the way maxed out.
for example, arkham origins actually runs and benchmarks faster with sli disabled and the second card set to handle physix.
far cry 3 only runs about 10 fps faster on 2 card than 1, but way less stutter with 1 maxed out.
crysis 3 sees huge improvements in sli
metro last light only maybe 30 - 40% increase.
IMO , i wouldnt get the dual gpu titan z, you are better off getting 2 780 tis in sli, but as i stated above, sli isnt always great.
try a single 780 ti for a month or so and see if you need more firepower after that. just keep in mind that sli doesnt always give you what your looking for
im an nvidia guy, BUT, the r9 295x2 looks pretty badass, but i would still go with the 2x 780ti in sli. they blow that r9 outta the water