I've generally made reservations when I've been there for dinner - so unless things have changed since I was last there, you can do so - and should.

Also, it's no more than a few hundred yards from the Taj Mahal hotel, but may be worth taking a cab if you're not Indian so you don't get accosted by the numerous beggars outside the tourist-heavy Taj hotel area. And once you get to Rhythm House, you're less than a hundred yards from it (although that's the tricky part in the narrow lanes).