Just to throw a spanner into the works :)
I have slightly better d/l speed 12Mb/s but poorer upload. ( wireless laptop )
I don't have too many problems with YouTube
Although as Nev says, if you are paying for 20Mb/s then a call to your ISP to ask for an explanation is in order.
Be prepared for a stock list of excuses.
Try taking the 512MB module out and run on 1GB.
Swap them over if no improvement.
Put both back if no improvement.