Hi,
before considering upgrading to 3700s these seem to have reached EOS(end-of-sale) cycle.
Refer to this link for EOS on the 3700 series
http://www.cisco.com/en/US/products/hw/routers/ps282/products_eol_models.html
Why upgrading from an already EOL model such as the 2500 series to a EOS model 3700.
If your 2500 series is currently your Internet router I would recommend the 3800 series, and you will bring a new platform ready for voice,data,video, and other functionality.
If you currently have a 2500 series I assume your network is small with no more than
200 users, as far as bandwidth you did not specify type of traffic , voice, video etc,
but in any case if you consider using the 3700 series, these do support T3 modules and should handle what your current 2500 series handles.
See data sheet for supported modules on the 3700 series .
http://www.cisco.com/en/US/products/hw/routers/ps282/products_eol_models.html
See data sheet comparison,for the 3800 series
http://www.cisco.com/en/US/products/ps5855/prod_models_comparison.html
hope this helps
Jorge
Jorge Rodriguez