| Dave, halogens and dimmers.
First off replace the halogen bulb soon as.
Your halogens have to be compatible for dimming, usually this means an electronic transformer with suitable dimmer.
Your dimmer has to be compatible with halogens, usually this means an electronic dimmer with suitable halogens.
IF....you have used a standard dimmer with transformer halogens then you could theoretically have the halogens dimmed, although the life span of all components would be questionable. In other words, summats gonna go sooner or later.
The wattage of the light unit is the same as the sum of the lamp wattages. Suspect you have four 20W halogens which would mean 80 watts total (wattage load is same in low voltage or 230v mains), or 4x50W = 200W.
Some dimmers have a minimum load of round about 250W to 400W - check your new dimmer.
There really are different types of dimmers, so the new one you bought must be confirmed as compatible with halogens.
Also, some dimmers require a neutral connection - does your new dimmer need one and is it missing?  | 
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