Actually, the photos you show, as you show them, aren't too terrible. If you want to invest in something to help improve things however, a different lens isn't going to do it. Not for $250. Not unless you want to go full manual focus. Faster primes can be had but of course you lose your ability to zoom in and out. If this is something you intend to do often, save your coins and lay your hands on a Tamron or Sigma 70-200 f2.8. I see them used quite often for around $600. You'll probably also want a monopod or tripod to use it with. In a venue like that, the monopod would probably be the Allowable option.
I think in the interim however, a decent editing software package would take you miles further than a different lens (in the next week).
Set your camera to shoot RAW+.
Set your camera to use M mode.
If ISO 3200 is working for you (can't tell in the small photos), leave it there.
I would leave the camera set at f8 or f5.6 to maintain constant aperture. Change it if need be but in my experience, most consumer zooms are a touch soft wide open at their extreme ends.
Adjust shutter speed as needed to get a decent shot. Panning (following the action with the camera) can help once you learn how to do it but definitely keep the shutter speed at 1/200 or faster.
Here's a tip, when you have a lit white wall as you do in the arena. Set your camera to Spot meter. Aim the camera at an evenly lit portion of the wall. Set your shutter speed for that (use a distant area to keep things constant) and Slow it down by 1 stop or leave the shutter speed and Open the aperture one stop (f8 - f5.6). That will get you pretty close to proper exposure for the environment.
After you've taken your photos, head over to Adobe.com and download the trial (30 day) of Photoshop Elements, whatever the latest version is. For your money on a pay to use program, that's probably the least expensive and best for you. Load your RAW files (either PEF or DNG depending on how your camera is set) and from there you can adjust exposure, white balance, sharpness, etc. You will get a much cleaner result that way than editing a JPG (especially with Picasa - I'm Feeling Lucky
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