The fastest film I have ever shot is Kodak Max Zoom 800. It was pretty grainy and lacked detail. Just about any pro shop stocks Ilford, Fuji and Kodak ISO1600 and ISO3200 speed film, but it is usually used by people doing art work or severe pull processing. Most all photojournalists still using film use ISO800 as its grain is no problem for newspaper print. however, most are now using high end digitals like the Canon D30 or Nikon D1.
You should try some Fuji Provia 100F slide film for daylight shots. The color saturation and incredible detail are amazing. I have shot some motorcycle racign and you can see the riders face clearly through the the visor at 300mm zoom. It is more expensive to buy, cheaper to develop, but more costly to make prints from. Here is a link to a shot from the roll of Provia 100F. This is one of the best shots I have ever taken of racing.
My Canon EOS Rebel has trouble autofocussing at 5.6 or higher. It will hunt back and forth and I will miss the shot so I prefocus and manually tune as they pass. It takes practice but it offers me my best chance it is quite a challenge.
I am hoping to purchase a couple of items next year. My Tamron 35-80mm lens has quit on me and I am going to buy a Canon 50mm f1.2 prime lens to replace it. For US$89 and it still rates extremely high for sharpness. The other is a long range flash so I can fill the shadows in daylight shooting.
Aysedasi, do you use flash at all?