What do you mean EXACTLY with "you can't change the resolution"? Does it mean after choosing the resolution and pressing the apply button there is just a flash and nothing happens? In that case there should hopefully be some error message in the ingame console (press shift-escape to open it and see it.. escape will close it again). Or something else? Sometimes when the desktop resolution was set to 16bit while one tried to switch to a 32bit (nexuiz default) resolution it was also not working..
Does it crash when trying? There was a bug in the ATI drivers that made them crash on video restart (when you press apply) and sometimes on closing Nexuiz. Either wait a few more days for the next version of Nexuiz which has a workaround for that or try installing the latest ATI drivers which got fixed.
Also its a very good idea to at least mention the exact graphic card you have.. not just a good one. And the operating system you are running.. remember nexuiz works on linux, mac and windows. That will really help us helping you
