Where should i start if i wanna modify the FPGA code on N210

Hi, I am currently doing channel sounding on USRP N210. There is an
example of sounder in the old release 3.4. However, it is designed for
usrp1. I am reading its fpga code and trying to follow its work flow.
When doing grc based channel sounding, i can not work at a high
sampling rate. Also my network card is Intel 82579 gigabit card which
seems to have some bugs. It overflows even at several MSPS. So if i
wanna modify the FPGA code for N210, where should I start? Also i am
wondering if there is some ram on the FPGA of N210 so that i can
temporarily cache the data on FPGA. Thanks very much.