I would skip the inverter (12V dc to 120V ac) and get a laptop converter. This one has worked well for me. You select the output voltage between 16 and 20V.
Cal
http://cgi.ebay.com/Universal-12V-16-18-19-20V-Power-Adapter-Laptop-/350357709559?cmd=ViewItem&pt=LH_DefaultDomain_2&hash=item5192f262f7
One reason to do this is that it is probably more efficient than using both an inverter and a laptop power supply. This converts DC to DC and avoids the additional inefficiencies of going from DC to AC and back to DC.
This would be a good purchase for someone who would run a laptop off a house battery for an extended period and is concerned about power.
However, if you have a large laptop, you might find that they don't make an converter that can sufficiently power your laptop (as I did.)
Also, I would not hesitate using a cheap inverter to occasionally power a DAD laptop. After all, most cheap uninterruptible power supplies designed for computers employ modified sine wave inverters!
Over the years I have had three MSW inverters fail on me, purchased from Target or Best Buy. If you get a cheap MSW inverter, don't expect it to last too long. Also get one that is rated 2 to 3 times the wattage demanded of the laptop power supply, otherwise it may shut down when it gets too hot. You also may find that the inverter shuts down because of the voltage drop though the cigarette lighter wiring when the van is not running. Finally, if you ever plan to power AV equipment off your modified sine wave inverter, expect the harmonics from the 120V square wave it produces to be audible in the sound from your TV/DVD or component sound system. Also don't expect a MSW inverter to provide sufficiently clean power for an uninterruptible power supply.
I actually am using three inverters in the van at the moment - a 2000W PSW inverter charger, that charges the bank and is tasked to invert only when powering a microwave oven; a 300W PSW inverter for AV, computer stuff, and most everything else, and a 1550W MSW inverter that powers a dorm fridge (granted, this is grossly inefficient compared to Dan Bertko's 12VDC fridges, but works well enough for road trips and half day outings when I can plug in after parking and is much more efficient that the 2KW PSW inverter.) All of these are powered from the same battery bank. This bank is usually disconnected from the vehicle charging system unless the engine is running and I know that the bank is not so discharged that it might draw too much current from the alternator.