If you have any ventilation, I would imagine you could have at least a 250-watt light in there without any trouble. Heck, it'd probably be in your best interests to raise the temperature by ten degrees.

If you plan on doing another grow, you may want another fluorescent light anyway, for vegetation. That would work a lot better if you had more plants too, since a lot of light is wasted from a 4-foot tube when you only have a few plants.

You're not really from the Netherlands are you?