I would suggest you use Illumicare LED MR16 lamps, 6watts each. the 3000K at 45 Degrees will give you a nice soft, even distribution and at approx 240 Lumens each these are as bright as a 20W Halogen MR16 lamp. If you find you want it brighter then bump up to the 2X LED MR16 lamps which provide nearly the same output as a 35W Halogem lamp. Much will depend on the ambient light conditions, the reflectivity of the object and just how bright the desired effect needs to be.
The nice things about using LED is the relatively wide range of input voltages that they will accept. This makes doing your voltage drop calculations much easier and it also allows for the use of significantly small transformers.
Hope this helps.