I've used the lightest 26" rims available, 32 spokes, and still have had no problems. What's more important is the quality of the build. If you want to play it on the safe side buy new rims and have your wheels rebuilt by hand. It would be economical because you would not have to be buying new hubs.
I've use the lightest Mavic racing rims that weigh around 400 grams and have had no problems touring or mountain biking. The thrashing they take on the trail far exceeds what they would go through on the road.