You could take a wired output from the modem into one of your PCs and use the second PC with the wireless card. Your modem would need to have more than 1 RJ45 port (1 for direct connection to PC1, another for the wireless router).
I'm not really qualified to comment on wireless and gaming but my son has wireless in his room and runs some pretty quick on-line game programs without any apparent problems. He has a 108MBps wireless adapter in his machine.
Wireless signal attenuation does depend on the structure of the house. The thicker the walls (older houses), the greater will be the signal loss. This can be a problem in large, old houses. There are some adapters available that claim to have increased sensitivity and therefore greater range, but I have not tried them. AS long as your adapter utility shows the signal strength as better than "low" then you should be OK.