2x15, or 2x30 both sound good to me.
2x15 gives you the option of getting half the voltage for er, something...?

Bear in mind that due to the voltage drop from the bridge rectifier, and the transformer regulation (and numerous other effects) you will get less than root 2 times the rated voltage. I think you'd get away with a 35V transformer (e.g. http://www.rapidonline.com/Electrica...nsformer-82719). Worst case scenario is you have to reduce the voltage a bit. That's easy enough - just wind a few turns of THICK wire on to the transformer and connect it in series with the output. If you put the wire the 'wrong' way round the current induced in it is 180° out of phase with the main winding, so it opposes the main current and reduces the output voltage. Side effect is that piece of wire gets a bit toasty - hence the thickness.