If you have a 3D modeler around, try modeling cacti by using cylinders and modifiers/splines to bend them in shape. The easiest way to take care of the ends is to extrude the cap of the cylinder, scale down the new cap, and repeat until the ends are rounded. As for texturing, I don't have many ideas on it, but you'll definitely want to take care of the little needles by texturing. Modeling out the needles will take way too many polygons, though you could use planes and alpha textures to create flat, low polygon needles.
Learn some 3D modeling. It'll get you far in NL.