Welcome to Nikonians! Unless the gyms you shoot in are extremely dark, the D700 should be more than up to the task. The 24-70mm that Alan mentioned is a good choice. You might want to consider a prime lens like the Sigma 50mm f/1.4 EX DG HSM or the Nikkor 50mm f/1.8G AF-S. the Sigma is a about 1/3rd the price of the 24-70mm and the Nikkor is about 1/8th the price. Good Luck and Enjoy your Nikons!