What's your budget? If I know what you are looking to spend, I can suggest a body that will give you the best bang for your buck.
Yes, I'm a die hard Canon guy, and a full time professional photographer. Here's most of what I carry when I am working a sports event like Baseball or Football. Add a flash unit, a 1.4 tele-extender, a 2x tele-extender, a shooters vest, a two camera harness system, and some trash bags (in case it rains they make excellent disposable covers for the gear that I can use and keep working.) and you'd have my basic sports rig.
I agree with Saggs, Canon and Nikon are the way to go if you want to get even more serious at some point down the road, simply because of the wide variety of lenses and accessories.
I personally will never recommend a third party lens. I've had Tokina and Tamron lenses in the past, while less expensive then Canon lenses, they are quite sub par in comparison. The difference between a Tokina 300mm f2.8 and a Canon 300mm f2.8 is quite stunning. I literally gave all my Tamron and Tokina lense away to my sister-in-law and replaced them with Canon glass.
Something you might want to look into is a macro lens and a set of extension tubes. You can do some really cool close up flower shots. Here's a photo I took a couple years ago with a 100mm Macro lens of a gerber daisy. I love the color and that you can see the pollen. Forgive the quality a bit, this is a scan of a print (my scanner is crappy and old), it was easier to take it off the wall in the bathroom then search the archives for the digital file.
Nothing you buy today is going to take your current lenses, unfortunately. So choosing a brand and the direction you want to go is very important for you now. For me, I'm a Canon guy and if I didn't want to be a Canon guy... I'd still be a Canon guy. There's probably $15000 worth of gear in that photo above, maybe more then that if I do the math. Switching systems now, would be very expensive for me (Not that I want to or anything) , and you should see all of the other stuff I own on top of whats in the photo.
I love my gear and I'm looking forward to the new high speed sports camera that Canon is bringing to market. Fourteen frames per second and ISO speeds that are essentially night vision!
The important thing is to choose a system, whether it is Nikon or Canon or whatever, that can grow with you as your interests and skills grow. Like Saggs says though, the bodies get better all the time. Your real investment is your lenses. Always buy the best glass that you can for a given application. Your lenses will transfer to the latest and greatest body when it comes out.
I've had that 300mm f2.8 for almost 10 years now. I've worn out 2 or 3 shutters on at least 3 different camera bodies in that time period.
Of course, I'm not saying you should run out and drop 15 grand on L series lenses and top of the line bodies. I'm saying you should pick a system that you can get into for whatever your budget is, but as your interests evolve, you can upgrade to better glass and better bodies one piece at a time as you can afford them without ever having to completely change systems.
I did my first wedding with a Canon A2e (35mm film), a Tamron 28-105 f2.8 lens, and a flash unit (I can't remember which model I had). Did my first sports with an old Canon AE-1 with an auto winder on the bottom and 70-200mm manual focus lens. Now I shoot a Canon 5D Mk2 and a 1D Mk3 with all L series lenses. I slowly upgraded stage by stage over the years to what I use now.
On a photography related note... Here's a photo I took yesterday. A local High School softball team has a player whose mother is fighting breast cancer. They hosted a tournament yesterday and wore special pink uniforms for the event. When the tournament ended, we did this shot of the girls standing in the shape of a breast cancer awareness ribbon. They're going to get it mounted on a plaque with an encouraging inscription and present it to the girl's mother before she undergoes surgery in a few weeks.
