The new Extreme will not really provide significant improvement with normal "n" devices, but the signal range will likely be a bit better on the new Extreme mostly in part due to an improved antenna design.
I would estimate the range improvement to be 5-10% as a result of some informal tests that I've done with the product here. No way to predict what your results might be, however.
If you are in the market for a new router, then the new AirPort is probably worth a look, particularly if you plan to add 802.11ac computers in the next few years.