To answer your question, yes.
The only reason to add a second card that is NOT identical to the first is for physics but for a high-end card that is a waste.
Add a second GTX280 in SLI or else get a low-end one just for physics (there's a chart).
I don't think we're very far from some new NVidia releases so you may wish to wait a couple months. You may want a new card or if not at least the prices should drop on the GTX280.