Hi and welcome!
First thing you should do is creating a Nexuiz shortcut on the desktop. then open it and add "+ exec low.cfg" to the command line and save it. Although your card isnt really so bad, it might help.
What sounds a little ominous to me is that you can't detect properly if you VGA is a Radeon 300 or 550. Are you sure to have proper drivers? If not, update!
Another thing you should really do is to get the game run in windowed mode for first. If you can enter the menue, set it to windowed mode. If you can't, you have to edit the config.cfg which you can find in the Nexuiz\data directory. Open it with WordPad and add this line:
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vid_fullscreen "0"
Save and close it. Then you are able to see error messages, for example from the windows firewall.
Good luck!