Are these 12 volt lights, or do they only work when you're on AC power?
Assuming 12 volt (DC) lights:
The LEDs will not dim the same was as your halogen lights currently do. Depending on the dimmer, the LEDs will go from bright to dim quickly, and then off for most of the dial, or they'll be unchanged for most of the dial and then dim / go out.
Depends on the LED replacement bulb as well. If you use LEDs with built in converters (aka the expensive bulbs), they won't dim and may be damaged by attempted dimming. If you're using generic bulbs from ebay, they may dim somewhat, and would not be damaged by the dimmer.
