Performance of the 770 SLI should moderately dwarf a single 780ti, however SLI comes with its own challenges. Dual card SLI setups will see between 40~90% of the second GPU's power being used depending on the SLI optimization of whatever game you're playing. Consider all of the following:
Pros:
-More power (sometimes significantly more) for less money.
-More easily accessible upgrades. You can buy one weaker card and then save up money for a second one over time. This not only allows you to more easily prepare yourself for a heavier investment, but also allows for possible driver optimizations and whatnot.
-Bragging rights.
Cons:
-Higher energy consumption. Could mean higher electric bills.
-Higher heat generation. Can lead to you requiring stronger cooling solutions for your other components and, in some cases, faster hardware degradation.
-Not all games support SLI.
-You must play games in Fullscreen mode only to take advantage of SLI; Windowed and Borderless Windowed modes are not supported.
-Must rely on both driver updates
and game developers to add SLI profiles and optimize them. Often leads to decreased gains in SLI compared to what you might expect.
-Requires compatible hardware. SLI only works if you have two of the same GPU (different models of the same GPU work, but it isn't recommended) and an SLI compliant motherboard. A lot of AMD motherboards do not feature SLI support. When you buy a motherboard, ensure that it includes SLi support if you plan to use SLI.
In most cases, I certainly suggest getting the strongest single card solution that you can afford, however it can be, as you said, economically viable to get one weaker card and then add a second one later. I love SLI and will continue to use it as often as I can in my builds, however I consider myself a crazed Gaming PC Enthusiast who spends entirely too much money on his systems, so mileage may vary.
Also have to consider what resolution you plan on playing at. Are you staying at 1080p, or do you plan on using a 1440p or 4K display (in the future)? I run a 1440p main display, so the dual 780s are actually being used most of the time when I play games. At 1080p, they would be overkill in a vast majority of titles. Dual 770s would do absolutely fine, as would a single 780ti, for many years of 1080p gaming to come.
