No question is "stupid" only un-asked questions. Yes...no impact on the wireless aspect of the device. The 480 mbps is the throughput speed from the USB device to the PC...most specifically the software/data it has on it....that's it. Your wifi speed is not impacted at all as it separate. 
On your 2nd question....theoretically yes, only in a perfect world would you achieve those speeds but as the documentation on all WiFi devices states; " reach speeds up to". 
Personal Example: I have 
NeatGear WNA1100 that is N format (150mbps max), I use this on my son's PC. It will reach speeds depending on how far from the wifi source, speeds any where from 65mbps to 144mbps. The faster the speed it gets specifically relates to how close I...