From you description as I understand it the booting of the W7 64 bit dual boot causes the problem? The same h/w when run as 32 bit XP works fine. Has to be the 64 bit driver then I'd say. Defective card or defective driver I'd guess? N speed networks will slow down to the slowest load they see. If mixed mode, my LinkSys N will drop from 270 Mbps to 130 Mbps as reported by the wireless applet. If I run the router as N only the 'offending' devices either can't connect to have a hard time sending stuff. In my case this was an HP Premium Printer C309g (N speed, don't let the G in the number fool you) and a Wii which is G only. In MIXED mode for the signal, the Wii can connect, but the network speed is cut in 1/2. The HP printer, well, I wanted N speed and it was returned. Unless it was put in G it hardly worked.
See if the NIC vendor has a driver update. If on the motherboard, consider getting a new NIC and putting it in and disabling the motherboard one? If it is a card, switch it with the one in the other wired XP machine and see what happens?
What I don't understand is how a wired card could effect a wireless signal?
Taking a different approach, I wonder if the W7 is causing significant network traffic? Essentially overloading the network? How/what are you using to determine network speed other than PING? Have you looked in W7's Event Viewer to see if there are repeated errors and warnings from network components? Is LLTD installed on all XP systems? If not, install it from HERE, does that help?
Also check out THIS MS link to troubleshoot network connections, the network slow one specifically. Could be a W7 card property setting? I guess you could boot both systems and compare the card properties, copy on paper the XP boot and then boot W7 and check them?