In general, simpler, slower modulation schemes can be received more reliably in worse conditions (less signal strength, more noise) than more complicated, faster modulation schemes.
In general, N uses faster and more complicated modulation schemes than G, and G uses faster and more complicated modulation schemes than B.
For best balance of rates and range, leave G and even B rates enabled even on your N-capable gear. But still upgrade all your Wi-Fi devices to be N-capable, so that they don't waste airtime by using G or B rates when they're close enough to the AP to use N.
Special radio modes that disable legacy G and B rates for the fastest possible N performance do so at the expense of range, and really only get a very minor performance boost out of that trade-off, and only at close ranges.
In some cases, the N-only optimizations can actually make your N gear less robust in the face of interference, especially interference from legacy G and B devices in radio range of you (e.g. your neighbor's gear). Basically you can get a small performance boost at close range if all of your gear does N-only stuff that only other N devices can see, but it means that any G or B gear in range can't see those transmissions, so they accidentally transmit at the same time and clobber your N-only transmissions. Turning off N-only optimizations can make your N transmissions more visible to legacy G and B gear, so that they stay off the channel while your N clients are transmitting.
There is one thing to be said about channels. N gear can be configured to use only legacy 20MHz-wide channels, or it can be configured to support both 20MHz-wide and 40MHz-wide channels (the 802.11n spec does not have a 40Mhz-only mode, just 20-only and 20/40). When you enable support for 40MHz-wide channels, some transmissions will effectively use two contiguous 20MHz channels. That means that if you upgraded from a G AP on channel 1 to an N AP on channel 1 in 20/40 mode, some of the N transmissions would use not just channel 1, but channel 5 as well. And since the channels overlap quite a bit in the 2.4GHz band, that means your transmissions fully or partially overlap every channel from 1 to 10. So by using 40MHz-wide channels you have to worry about how interference-free you are on a lot more frequencies than before. If you suspect this is a problem, you could reconfigure your router to only use 20MHz-wide channels. It would mean you can only get 144 megabits per second signaling rates instead of 300 megabits per second, but that might be a good trade off if you're running into interference problems on wide channels.