What are the NAS boxes? PCs or appliance type dealies?
Not worth it, honestly.
Not only does your PC have to be enabled for such speed, your network devices to and not JUST the network connectivity but the actual storage media also. The bump in price to 10GBit and/or Fibre is stupid. The added complexity of LACP is not worth it.
Options for what? What tasks that you throw at your network are slow at the moment, and what is the model of your NAS?
Well anything more than 4 x 1Gb links from your PC is going to be wasted if that's all you've got at the NAS ends, approximately anyway. Quad port gig card, aggregate the links and do the same with the 2 at either end on the NAS boxes. Even that is most likely overkill.
I think your dual port NIC is the way to go. I recently picked up an HP 1910 24 port layer 3 switch with PoE and a few other toys for £75 on the 'bay, something like that should see you right.