You can store a large number of waypoints in the Nuvi 350 - probably several thousands (for instance, I've downloaded waypoints for all speed cameras in Sweden into my Nuvi 350).
The limitation is the number of waypoints that you can use in a route. Unlike other GPS receivers, the Nuvi 350 does not allow you to make complex routes (e.g. go from waypoint A to B and C to D). It only allows you to select a destination waypoint (in the "Where to" menu), and then it automatically calculates the route which it finds best to take you from your current position to the destination waypoint.
Optionally, you can select a second waypoint which is inserted "in the middle" of the route. To do this, when navigating to the destination waypoint, you go to the main menu and select a new waypoint (using the "Where to" menu). The Nuvi will then route you to the middle waypoint first, and then to the destination waypoint. But that's the limit, you only insert one middle waypoint in this way.
I've had my Nuvi 350 for a month now and i can really recommend it, it's a great navigation tool with awesome GPS receiver performance. I've also installed the GTM-10 (RDS-TMC receiver) in my car, and I can recommend that too - it works well together with the Nuvi (traffic disturbances, road conditions etc. are displayed on the screen and can be avoided).