The Layout Design Special Interest Group (LDSIG) rule of thumb for curve radius is here:
http://ldsig.org/wiki/index.php/Curve_radius_rule-of-thumb
If you read through my comments, you will see that with truck-mounted couplers (common in N) you MAY get to reliable operation, but no coupling or uncoupling, on a radius equal to twice the length of your longest car or loco. Testing of each piece in conjunction with other cars of various lengths would be needed.
2.5X should be pretty much achieveable with truck-mounted couplers under virtually all conditions.
Note that a scale model of an 80ft car is about 6.25" in N. Marginal operation would be expected at 12.5" radius (individual testing required), reliable operation with truck mounted couplers at 15" radius, and reliable operation with body mounted couplers would need the full 19" radius recommended by others.
Something to keep in mind is that an 80ft car in N is just as long as, and takes just as much room to turn as a 40ft car in HO. An 89 ft car in N is almost equivalent to a 50ft car in HO.
All of the above has nothing to do with looks on curves.
The NMRA's recommended practice for curves for both rolling stock manufacture and layout design is here:
http://www.nmra.org/standards/rp-11.html
Except for club and some N layouts, very few modern era layouts come close to complying with the RP. But few modern era and passenger car modelers realize that they really need to go up so much in radius to achieve to operation and looks they desire.
my thoughts, your choices