Time capsule file transfer

No, you can't do the initial transfer with the USB. The USB port is only to connect printers and hard drives to share. You'll have to do the transfer wirelessly or with ethernet.

I wouldn't install a gigabit card on your PC if you're only going to do the transfer once. It may take longer, but if you leave your PC running until the transfer is done would be cheaper than installing a gigabit card. You also have to keep in mind that the bottleneck for the transfer speed to the internal drive would be at the USB controller of the Time Capsule, so you'll probably be maxing it out even with a 10/100 ethernet card anyway (I don't know the exact numbers, don't hold me to this).
Edit: I just realized you might mean that you're pulling off the files directly to your Mac and not to the internal drive. I still wouldn't bother with a new gigabit card though.

And lastly, I'd do what you suggested. Unless you NEED a guest network as well, there's no reason to get a dual band TC. I've got a first gen TC broadcasting 5GHz N only with a Linksys WRT54GS broadcasting mixed b/g. Just set the TC to Bridge Mode and you'll have one network with two wireless access points.