Why is my Wi Fi connection so slow?

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Ok I have a 54mbps router plugged directly into my internet connection, this then transmits the signal via wireless to my pc as I really wanted my pc in another room. Why oh why is my connection so terrible? I cannot even stream a you tube video without it pausing. If I was to connect my pc via a cable it would be so much faster, my connection is 10Mb via fibre optics. I would love to know how to make this faster as its really becoming annoying, even some webpages take an absolute age to load.
I have a Dlink DI512 router and a Dlink G122 usb receiver

 

[Viral]

Ok this is what seems to happen to my connection. After I have been connected to the internet for several hours it seems to be a lot slower, atm I am trying to preload Bioshock via Steam and its really slow, around 10kb/per second. I have realised that if I change my channel on the router then the internet seems to be running at full speed again. Is there anyway of changing the channel automatically. Or is it slowing down due to the large downloads?

 

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connection rate is 54mbps