RTS Pathfinding

A* works with an arbitrary graph (a set of "vertices" where you can be, connected by "edges" of differing costs). What kind of map do you have? I can't really imagine an RTS that doesn't have a grid-like map underlying it, but with any map, you can create your own set of vertices from knowing where the obstacles are (put a vertex a bit "out" from each corner of a polygonal obstacle), then add a few more vertices between pairs of these, add one where the unit should end and where it begins, compute costs between each pair of vertices (only once if possible!), and launch A*.

To sum up, any mapping system where a node has perfect knowledge of the nodes that it directly connected to is capable of supporting A*.

It's just a height map. The x and z coordinates are evenly spaced. but how would i determine what vertexs the unit can cross and can't cross. I was thinking i could check the y value of the two triangles that share the two vertexes and if both there normals y values are less that a certain number its not crossable otherwise it is. Would that work?

This should be easy: penaltize going up in height, and reward going down in height. You could find the "height" of a node by finding the average hight that tile.