I'm considering starting martial arts training to help get myself back into proper condition.
For conditioning, you can't beat kick boxing and tae kwon do. In a street fight, kick boxing is much more effective, and in general a kick boxing workout will tire you out a lot quicker than tae kwon do. Tae Kwon Do has it advantages (flexibility, better concentration, beautiful kicks...), too, but it requires a lot more dedication than kick boxing.
I'm wondering is there one form that is better than the other? I don't know allot about martial arts other than what I've seen on Kung Fu movies, but I hear it's good training for tennis. I hear you can get some training videos too, Has anyone taking Mrtial arts? and what is the best form?
Overall, I think the speed, explosiveness, strength, balance, endurance, and power you develop in kick boxing will translate pretty well to the tennis court. The real advantage in the case of tennis is the hand speed and coordination you'll develop by boxing that you can't from Tae Kwon Do. The latter isn't great for hand techniques.
If you want kung fu, I recommend wing chun. It's pretty good in street fights and it develops incredible hand speed and feel, great for volleys. Kenpo, or Kempo, Chinese or otherwise, is also great for developing hand speed and power. But I can't vouch for Kenpo having never taken it.
If you want to kick ass in a one on one street fight, like Scott said, a grappling style is best. But it won't necessarily get you into better shape.