Duki, I had a 980ti g1 which I just sold in anticipation of Pascal. I now use a GTX660 with mixed success. The 660 still pushes over 130FPS in battlefield 4 with most settings on low, making it very playable. I have a 144hz 1440p monitor which has over 80% more pixels than a 1080p screen.
Most people say the 970 is for 1080p the 980 is a bit of an overkill, and the 980ti is definitely an overkill for 1080p, i.e. you will be not getting your card to work fully as it can pump out too many frames. I found every game to be very playable at 1440p and the resolution increase is amazing. Getting a 900p monitor for a 980ti is a complete joke in my opinion.
I have played the witcher 3 with everything maxed out at 1440p and it does not go below 60 that I can remember. On 1080p it would be much higher than that, but at some point your vertex calculation overtake your pixel pumping ability. Remember, the 980ti is made for higher res than 1080p. You will be able to game long with 1080p. 900p on Ultra will look less good on a 20" than 1080p on High on a 23". You should in any case consider getting a 144hz monitor because every frame above 60 is wasted, and it makes a huge difference in gaming.
It is amazing that you even consider a 900p out of fear that it will outdate your graphics card while 900p is really outdated and it was outdated for many years now.
my 2cents.
Gabor