Not very much you can do with the PIX515. First, that device is to slow for more then one FE-interface (assuming 100 in and 100 out).
But you could try to spread you traffic over multiple interfaces. If you have one DMZ at the moment, you could connect your servers to multiple DMZs witch their own interfaces.
Before you start, you should do a baseline to see where your bottleneck is. The interface that is saturated has to be devided into multiple logical interfaces. But only if these systems don't have to communicate with each other.
All in all, buying a new 5512-X is probably cheaper and makes more sense then investing work in a platform that is now eight years end of support.
