Once 3-4 Years has gone by, you will probably be looking at medium settings to get playable frame rates, and that's being optimistic. Of course, it depends on what kind of games you are trying to play, but that is just my guess based on how my GTX 680 has aged. Nobody can really know for sure.
As for the 800 series, latest rumors are that the high end cards for the 800 series won't be seen until early 2015. On top of that, we don't even know if they will be on 20nm architecture. This is due to TSCM not being able to get it's ******* act together and produce fast enough. I am still hoping for the best, but at this point, I really don't know.
Price wise, usually when new generations come out they replace the price of cards before them, meaning theoretically the 880 should be the same price as the 780. However, this has been changing lately. Nvidia has been releasing cards that appear to be high end due to their naming scheme, like the 680 and 780, but then outdo those with the REAL high end, for example the Titan and the 780ti, respectively. The really high end cards ended up being much more expensive than previous generations were.
So, in the end, we really don't know. Just know that something new is always coming out, don't wait forever. I'm waiting because I already have a GTX 680, and it does almost everything I need it to. I looked, but maybe I missed it. Do you have any kind of GPU now? If not, buy a 780 or 780ti. It's not worth it to go without a gpu waiting for something that is possible 9 months away. Now, if you had a pretty decent GPU now, I'd say wait, but it appears you don't have one.
Since you are gaming at 1920x1200, you really don't need a 780ti. A 780 or 770 even would be fine. However, none of them, even the 780ti, will max games out for 3-4 years.