If your trying to keep your budget low the Cateye LD600 is actually brighter from the rear and the sides then the 500. Also take a look at the BLT http://www.blt-lights.com/rearsuperdopplerdx.htm

This is actually better then the Cateye from the rear, but I don't think the side illumination is better. The BLT is only about $5 more then the 600.

Later when you can afford it you should buy 2 more rear lights and form a triangle with it. This triangle shape is less confusing to drivers then using just 1 light, thus you stand out far better.