Hi Todd,
You've got a good start on the road to "Hail Mary" lighting conditions!
It takes more tweaking these nose-bleed ISO images at the edge of reason that those taken at "normal" times of the day.
I do a lot of shooting at high ISO settings. For me many times it is either crank-up the iso or I won't get the shot. Sometimes, it's a matter of shooting a lot and hoping for a few good pix like what it appears you are doing with your night dirt track work.
First thing is to always shoot in raw so you can clean up the noise and all else in post since lighting, color-temp, etc. is always beyond your control.
Generally, I'll clean up most of the noise at this point. Many times, I've got to crank up the exposure even higher that the original exposure, sometimes 1-2 stops for one of my cameras. I'll generally push up the shadow sliders and pull down the highlight to contain the contrast as best I can so that the shadows don't clog and the highlights, other than specular (headlights, stadium light) don't go blank.
Sometimes, the sliders in Adobe Camera Raw/Lightroom aren't enough. You can then try some "local" adjustments that can be used "globally" and even get more from your raw processing.
For noise reduction in raw files, it's always a balance of just enough and "grain" (noise) vs. too much correction and a mushy (soft) image. Some cameras just need luminance NR and others need that along with color. It all depends upon the camera.
If I can't get all the noise out to my satisfaction at this point, then I use a plug-in. I like Nik's Define, though Topaz DeNoise also does a good job and I use that with files from one of my cameras since it works best with it. There are also several other noise reduction plug-ins out there. Try several and buy what works best for you if ACR/LR isn't enough.
One thing I've found with Define is that it does a great job with little effort but is usually just a little too aggressive for me. Since it puts the cleaned-up image on its own layer, it's just a matter of blending with the layer(s) below, usually at 80% opacity to let some of the noise come through for the illusion of image sharpness.
Now regarding noise, pull too much out and the image goes to mush and looks plastic. Too many people that never shot film think they've got to clean it all up. Don't. Even Tri-X at ISO had grain at ISO 400. Sharp grain gives your eyes something to lock onto and get you an illusion of sharpness even if the image is a little soft from missed focus or movement.
Color can be metric and tuned to the 9s or can show the grit of ambient which IMO, adds to the action at hand. Your eye will always correct through your brain. The image needs your judgment to balance it to your liking.
If your camera is fairly new (made in the last 3-4 years), ISO 6400 may be just the low-end of what will work for you. With good craft and careful processing, I have pushed my cameras their max of ISO 25,000, the highest most of mine will go and even to ISO 50,000 for another which still has headroom on the dial. Don't be afraid of pushing up the dial and seeing what you can tolerate! If you are shooting a high megapixel camera and just putting things on the web, you can get away with an even higher ISO when you downsize your photos for the web and get better quality.
If all you need are for small prints (8x10 and smaller) or for the web, then don't be too concerned about shooting at high ISOs but do testing first to see how you like the results!
Your personal high ISO limits all depends upon the subject and your tolerance for image quality. For me, I'd rather have a noisy and marginal color image for some of my work than none at all or something that needs lighting disrupting both the environment and scene in front of me.
