vid_netwmfullscreen 1 in the console (shift + esc) solves your problem with streching the fullscreen game to two monitors (vid_restart is needed after you set the cvar).
Unfortunately, this is undocumented, I've already mark this on IRC to fellow devs.
Also, to my experiences this makes the game to loose a few FPS, but not too much, 10-5 in avg, and, it *might* crash on wm events, but blame this on the bad NVidia drivers, not on the game (I've had several segfaults in the past, however I keep my drivers updated, and haven't seen a crash in a while).
8400GS should be enough to run the game at least on normal video settings, +blurry reflcetions enabled.
I'm liking the work you guys are doing on the models/textures. Will take a look at it when I get home on my asus g74 w/gtx 560 2gb. I'm a big fan of the Nexuiz, and this looks like that with better models, so I think I'll fall in love quick. It's insane what can be done on the darkplaces engine really, between the community and lord havok, they took that 1995 quake engine and turned it into a BEAST. They have done more with that than what they have done with quake 2 or even 3 to be honest. Though I do have to say the effort people have put into the doom engines are quite commendable to update those games look and feel to be much more modern than their 1993 counterparts.