Part 1 for people who don't want to go into file modifications :
I'm not sure if this has an impact on the actual resolution but :
Go to the options control panel, gameplay, advanced options (bottom). You then go in windowed mode.
You can move the window to see the whole advanced options available if you want to check them.
Anyway, in the game window you now have a top menu bar : file, window and help. In the windows drop-down menu you have various modes to select.
Also for people configuring the keys, be patient, it takes a few seconds after hitting Enter for the game to allow you to take the new input.
Part 2 for people who are ready to make file modifications (and already have done with UT99 or 2004, since as far as I remember, HG3D is based on a UT engine) (needs a lot of further testing and feedback is welcome):
I think everything is in hg.ini (system folder).
You can change your input keys easily there.
As for the resolution :
I changed the original settings 640*480 to :
ViewportY=800 (widescreen laptop explains the unsual resolution)
I don't feel an actual change on graphics but the game now plays at a decent rate (before I found it played too quickly, you could be shot to death before seeing where the enemy was)
I switched ScreenFlashes to False because I think it changes the flickering that sometimes happens in the select screen at the beginning.
Hope this helps.
As for me there are a few things I'd like to have an answer on (I'll investigate but if anyone has a solution) :
Why does sometimes saving makes the game crash ? Specific places ? Limited number of saves ? in hg.ini I saw there was suppose to be a quickload/quicksave feature, I'll try that.
Is there a way not to have some "cracking" in the sound effects ?
Part 3, the most important : Thank you FromWithin