I use halogen mr16 and mr11 lamps for mountain bike lights and although they're specced at 12v you don't need to worry about running them off a car battery that may kick out ~ 15v on full charge. In fact they become significantly more efficient at higher voltages. I deliberately run mine off a 18.5v li-ion battery and haven't had any problems to date.
If you go with mr11 you can get 5w bulbs which should be more than adequate.
As has already been pointed out, the problem you will have is heat.
I also have an led mr16
http://www.ultraleds.co.uk/leds-seoul-c ... -1630.html but they aren't cheap and not as bright as halogens despite what the seller usually likes to tell you. That said they aren't too bad. If you choose to go for the cheaper mr16/mr11 leds be warned they don't really live up to expectations.
If you're handy with a soldering iron you could try modding you centre cabin light with some cree xr-e's or soeul P4s like this
http://www.taskled.com/install.html but that's geeky
and will lead you down the path to the
candlepower forums so beware!
Is it possible to fit those little dichromic? spotlights that use a transformer to step down to 12v and wire them to use power direct from the battery when not hooked up but then work on 240v through the transformer when on hookup? Suddenly I feel tired...... 
