Are you gaming on your "mini lan" pc? When you say "mini lan" do you take it to lan parties? The GTX 280 is still a good card and DX11 isn't really ramped up yet, you still have a long way until most games support it. DX10 still looks great.
Or wait I just noticed you said help contribute to a 470. Do you mean that the 470 would go in the "mini lan" pc? If so, i would just stay with the 280.