We are arriving from Rennes to Paris at Gare Montparnasse in August, 2011 and need to make a train connection to go from Paris to Amsterdam from Gare du Nord. We need to debark from one station in Paris on Sat. at 1:20 PM and taxi with luggage to another station for the Paris to Amersterdam train. Is a 2 hour difference enough time, or do we need a 4 hour difference?
Two hours is more than enough time, unless you have other plans while in Paris.
It's a switch between Montparnasse and Nord. The taxi should make it in a half hour, even if the traffic is a bit rough. Figure another five minutes at each end getting from the station to the taxi and vice versa. There's a single metro line that connects the two which will be cheaper and maybe a little quicker, but will require a bit more walking.
It's an 18-minute ride on metro line 4 from Gare Montparnasse to Gare du Nord. If you're packing light, it's easy. Trains run about every 5 minutes. You could be on the train and gone by the time you get to the head of the line at a taxi rank at Montparnasse.
It's a 500 yards walk from the platform head at Montparnasse to the Metro station. Depending on the luggage you have, that can be a challenge. However, as you have plenty of time, that shouldn't be a problem. You will pass right in front of Paris' most famous skyscraper office building, the Tour Montparnasse.