I would recommend to connect to your 555 a smart MOSFET that Infineon makes. They are protected against overcurrent (if your lamp goes into short-circuit), against overvoltage, overtemperature etc. Infineon do have many to choose from that will suit your requirements on low conducting resistance of the MOSFET.
Also, keep in mind that if you will have long cables to your lamp, then they will act as inductanses and you will get high-voltage spikes over the MOSFET when it stops conducting. If using a non-smart MOSFET you will have to take care of that possibly problem.
Recommended switching frequency for the PWM would be about 60-120 Hz. If using much higher frequency you will get too much switching losses in the MOSFET. If using to low frequency your lamp will flicker. Other makers of smart MOSFETs are ST, Philips.
/Pim