Depending on how far in advance your trip is going to be, you can book a discounted room through either www.wotif.com or www.ratestogo.com

You can as previously mentioned, check out reviews on www.tripadvisers.com

www.booking.nl is another great site for discounted rates.

I have always had a double bed, not two singles pushed together in amsterdam, even when staying solo.

Have a wonderful trip!