Haven't been playing for 4 weeks now since I dont have the kind of line I can play on where I work (which blows), but I've got the exact same setup on my comp, just a HD 5870 insted of your 280 card. I can tell you that I had no problem what so ever 5-boxing with that setup 4 weeks ago, in fact everything was smooth as silk.
I can't help you any more I'm afraid, but I can't imagine your problem being in the hardware department (unless something has broken or you're not using the correct drivers).
Hope that helps...
/Sorry
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