Do you know a reasonably priced downtown hotel in Amsterdam?

Depends on what you mean by reasonable ;-)

These are pretty good search engines for Amsterdam:
http://www.bookings.nl
http://www.hotelres.nl/

Amsterdam House is a nice, 3-star hotel, rooms starting at 75 euros. Also check out apartment options if you're travelling as a group: http://www.amsterdamhouse.com/hotel.htm

An even cheaper option is Hotel Aspen, a single at 35 euros if you don't mind a hall shower: http://www.hotelaspen.nl/

And these guys seem to have way too much fun, so it's bound to be a great hotel: http://www.hotelprinsenhof.com/
standard rooms (shared shower): 45 for one person
with shower and toilet: 80 for one person

www.hostelbookers.com

Ive used this a few times when going to amsterdam