I have recently moved my router (Airport Extreme) several rooms away from my computers (i-Mac and PC). The i-Mac is fine and is accessing the 'net with speeds up to 30Mbps (uses wireless-N). However, my PC has a USB adaptor installed (Sabrent 802N), and is seeing low signals which appear to be limiting the network speed to less than 2 Mbps, and sometimes even slower. I have read where poor signal strength can affect speed (at least on a PC).
I may choose to replace the USB adaptor with something that is better in low signal areas. Does anyone have a recommendation?
Before you spend money, I would try the basics: raise the router above furniture level, relocate cordless phone base or video sender away from router or computer, add an extension lead to the USB adapter and if possible place it next to the wall nearest the router (there's some reason this helps and it's not just distance).
You should also try different channels -- some frequencies work less well in some environments -- but especially if you can detect neighbouring wifi. Move to a channel 5 stops from strongest neighbour.
not quite as much information as fihart, but from my experience pci wlan cards are a lot more stable, then usb dongles. Or at least other wireless ways other than usb i believe are better, but are maybe more expensive then usb dongles etc.