It gets pretty hot where I live and I have an older house that is hard to cool. Ambient in the room where the computer will be ~76F. Will I need to put an intake fan on my new computer that I just ordered? Not going to OC, will be used for casual gaming no extended sessions over an hour.
Just keep an eye on your temperatures when you get your computer. You can monitor temps with Speedfan and you can stress your system with Prime95.
If your CPU stays below 60°C and your system below 50°C during the torture test, I'd say don't worry about the extra fan. In the event you find you do need an intake fan, be sure to get a good quality, quiet, 3-pin fan since you are more prone to hear/notice a front fan. Look at the dBa ratings of fans and try to stay below 20 dBa.
+1 for letting your operating temps guide in deciding if you need extra fans.
The two fans you do have should be drawing in cool air from outside the case and exhausting the warm air out the back and top.