20 watts peak is how much it will put out before it blows. more than likely its probably around 8 watts rms(or constant). I wouldnt go for that bass. They're pretty crappy, and that amp is probably poor as well. Spend your money on a better bass.
http://www.rondomusic.com/ursa1rnbkpack.html This would be a much smarter and much better deal than that silvertone. Use your left over money to buy replacement strings I recommend Daddario nickelwound strings. and get it set up.
SX basses are cheap,but don't be fooled. They get very high praise around here. Excellent basses for seasoned player,and beginners a like.