Sounds like something your i5-3570 can handle with no problems, makes me wonder what software you're using...
Anyways, FPU coprocessors are a thing of the past, built around the 80's to help FPU calculations in 80486 CPUs, nowadays every CPU has an "integrated FPU coprocessor"
If you feel like needing more processing power you would have to invest in a dual Socket Xeon machine, and if your software supports it you could use Nvidia's CUDA or AMD's Stream to accelerate your FPU calculations (you'll have to buy an appropriate graphic card depending which technology your software supports).