I'm building a new PC soon and was wondering if I should wait for the 880/AMD equivalent to come out or just get the 780 now.
I would wait for the 880 and buy the 780 ti on fire sale
If you can wait as long as into late fall it could be worthwhile, but on the other hand if you have the money today then a 780 is a solid choice, or a R9 290 for that matter. Not much is known about the next series of cards, but apart from the regular rebranding of some existing chips we should see some new cards with relatively low power consumption at least on the Nvidia side, if that matters at all for the build you're planning. Otherwise, today I feel that the R9 290 cards (like the Sapphire with their own cooling) are still the best value.
As a general rule I'd say that it's always worth waiting a little longer /if/ you can, and then when one cannot wait any longer then get the best parts you can afford. Several games that will probably be good are coming out quite soon.
I am waiting myself, leaks indicate that the 870 and 880 should both be cheaper and more powerful to a good degree, a supposed leaked benchmark of the 880 shows it as being around 35 percent faster than the 780ti.
http://www.lazygamer.net/general-news/leaked-nvidia-800-series-benchmarks-show-massive-performance-jumps/