You will need to set static ip addresses on both the DNS-321 and the PC, a cross-over cable will work, but is not required - the gigabit port on the DNS-321 will "auto cross" if necessary.
No there are no settings on the DNS-321 that would make moving large files over wireless more reliable, you would probably need to tackle this from a wireless perspective - if your wireless card utilities show statistics take a look at the tx & rx error rates, you will probably find one of them to be fairly high, and will need to determine the cause and correct it.
Please note - if you are using an Intel PRO/Wireless card - these have a power saving feature known as PSP or power save polling that is not commonly used with consumer wireless routers and can cause problems of this nature - there is a work around on the Intel website for it.