[svandamme]

change channel
disable upnp on the router
set router and all wifi cards to long pramble instead of auto
don't use usb dongle wifi crap
buy better antenna's (stock usually is 4 DB, 15 bucks get's you 6-8 DB omnidirectional ones)
maybe consider getting an antenna extension cable so you can position it better on both pc and router
make sure you don't sit inbetween 2 antenna's, human=water, water is bad for wifi signal

last but not least, make sure no neighbour is downloading kiddy pr0n via your wireless... lock. it. down.