After reading many posts regarding this topic I've come to the conclusion that simply adding another route will enable my router to load balance all outgoing traffic over two xDSL connections -- am I correct, because this is what I must configure.
First of all, IOS will see two 0.0.0.0 routes and by default, load-balance outgoing traffic to those two destinations on per-destination basis. To enable per-packet load-balancing you should enable per interface 'ip cef load-sharing per-packet', but you propably don't want to do this.
The next problem comes from the fact, that if you have two xDSL lines propably each one of them has it's own assigned IP pool, to which you should NAT outgoing traffic - otherwise uRPF check on border routers of your ISP will drop it as spoofed. So you need to configure NAT with route-maps matching interface that outgoing traffic will exit the router (when NAT gets the packet it's already on it's way through selected interface, so 'match interface' action in route-map will match the one actually selected to push the traffic out).
The better solution for such issue is to use OER/PfR, which is dynamic version of this setup - traffic sent to external networks will be probed and if better path exists (better meaning with lower delay, via less loaded interface and many, many otehrs) outgoing interface will be dynamically adjusted.