Joshua,
I hope I understand your setup right. I would assign a static IP address on Main Downstairs NIC to a private address range that is different than the private IP address range of the router. If, for example, router DHCP assigns address in 192.168.*.* range, set Main NIC to 10.0.0.1 then all other downstairs machines' NIC's to get IP address , gateway etc. from Wingate DHCP. The wireless card in main machine can get its IP info from router. WinGate should be able to determine that Wireless connection is to the internet and deem it to be public and untrusted. This should work OK. I have a similar setup and no problems. See this link for a pictoral version which I think is similar to what you have
http://support.qbik.com/kbimage/Dslnetwork.gif and this knowledgebase article for further info.
http://support.qbik.com/index.php?_a=kn ... Networking
If it doesn't work, come Monday NZ time, someone smarter than me will be able to help.