Use one card.
Duel card setups (SLI and Crossfire) are for Computer Masochists.
Use them only if you enjoy self imposed mental pain, degradation, and humiliation.
Might be OT but that power supply has a strange way of listing its output. If I read it right 20a on +12 and 20 a on -12. Isn't that only a 240 watt PS on a single 20a 12 volt rail?
I'm not in yet but I am guessing that you bound your gun to a hot key instead of binding the shoot skill (or whatever it is called). Binding the gun probably simply takes it out of inventory and equipts it.