not sure if you want to play triple-A games like gta 5 or newer ones....etc. But your current graphic card(gtx 950) is enough for a 1920x1080 23", 24" or 27" inch monitor.
I recommend 1080 24" monitor as 27" on 1920x1080 the ppi(pixels per inch) is a bit crappy(means you can see pixilation).
But if you gonna play newer triple-A games, you might want to upgrade your graphic card to like a 970, 1070, RX 480 depending on the prices.
note: 970 being the most cost to performance ratio. I recommend waiting though since AMD releasing rx 480 at low price which might spark price competition from NVidia
Your current monitor resolution: 1440 x 900. You really need to buy a new monitor tbh.
Get an IPS(great for casual gaming, rts, movies, pictures....etc). Great colors, viewing angles and ok response time. You don't need those 144hz monitor or TN panels unless you are into competitive FPS games.
You can consider a freesync(AMD cards) or G-sync(NVidia) monitor which will actually help make gaming experience smoother. But its a bit more pricey fyi.