Even better we both shoot Canon, lol. Nikon is overrated and the inside flash or backlighting I think is cheating with some of the higher end models. Right now I have a 20D, 30D and 7D, where-as I started with a D30 and my partner had the 1D, which BTW has a 500 flash sync that no one realizes what a difference it makes and even at 4 MP it's one of Canons best cameras. Photography has actually gotten cheaper now with better glass and bodies. I keep a tamron 17-50 2.8, A Tamron 10-22 3.5 -5.6 and a 70-200 2.8 L second gen with a 580EXII flash and an off camera flash set for added light, the sigma 120-300 2.8 is coming along with the Canon 200-400 2.8 unless I find Sigma updates the lens they have, I love the Sigma and Tamron lenses, colors are richer and images out o the camera tend to be at least a meg larger than with Canon lenses because of the detail and color.
I'm not sure what the rules are there at the track, we are limited in Jersey a bit or at one track since it's where Kallitta was killed at E Town, other tracks are not as rigid, hell people hang over the walls which IMO is the stupidest move ever when shooting anything quicker than 10 seconds.
I feel the XTI is a bit limiting for use of making good photos without noise, even at 200 ISO it's apparent in low light situations which brings you back to glass, anything with a straight 2.8 is going to do wonders, even if it is expensive the difference between the 70-300 Tamron is night and day on both models you have. If your camera shop does rentals, try the L glass or similar 2.8 straight through, it's usually for me around 50 for a weekend, your camera store can be a big help, I have a great relationship with mine, rentals are no problem, if I need a 300-800 I can have it for the weekend. For what you want to do, please shoot in manual, AV or TV mode, all will give much better results, I shoot drag racing anywhere from 9.0 aperture at 500 shutter depending on light and drop down to 2.8 with an off camera flash filling in behind the car while hitting it from the front with flash on manual for night but have shot the 30D at night with a 2.8 and single flash but have to be close to the object and not having to go above 200ISO. The new 7D I just got will allow me to go to 1600ISO no problem if metered right and I nail it, but will still use a fill flash, the 580EXII will save you photos since the ETTL reads it back to the camera much better than earlier models, I would have gone with the 5D Mark but it didn't justify 2,000 more or getting new lenses for a full frame, I like the crop camera's just as good for what I do.
Now back to the noise issue, sometimes it's not noise you are seeing, it can be a combination of smoke and fumes wafting down track after burnouts even more prominent with a flash, I don't even bother taking the shot if there's smoke so don't blame the camera, it's the environment around the cars.
As for making money, you have to be dedicated, and of course track photographers will have issues with you stepping on their turf, meet the track owner, be polite to the other photogs and definitely don't critique them, they hate it since most of them are holding 5 grand worth of stuff in their hand, of course they know more than you and you're a new guy which makes it harder. I don't sell my photos at all and don't even own a printer, too much work for me, I make them available for file downloads at minimal prices, have been published in many magazines both large and small scale but doesn't pay well in any respect.
What I have found is racers like pictures of their cars, soft, in focus, out of focus, terrible glare and more, they just want a picture of their car, better if you get it right obviously too but these guys are not picky. Go out on the street and practice, a car going 50MPH is similar to a launch and will give you information for focal length, shutter speed aperture and lighting, shoot anything, since it's digital it's simply practice and basically costs nothing but your time. I still haven't found a reason for RAW images unless I feel it's just a bad day at the track and the images are suffering, JPG large in any of your camera's are going to be JPG's at some point when finished, it saves me on workflow allot.
Now lets get to "Being Different", look at any racing gallery and you see the standard "For Sale Shot" this is usually the car dead on, nothing around it launching and so on with a long lens, around 300 or so, usually the "Spray and Pray" group tend to overuse that 9FPS to "Not Miss It" again, cheating IMO. Compose a shot, wait until the right moment and take your best, refocus and give it another. Sometimes you need to do the "For Sale Shot", with sometimes upwards of 3 qualifiers, you have a chance to get three different angles, at least make one more interesting by having the crew, or composing the shot with onlookers or stands that have people in there, magazines and tracks love this. Empty stand shots are detrimental to the race itself so they hate them. Take a look at the cars on the return road, I began this years ago, and have had photogs really find sales increase with these simply because NO ONE ever takes them, dummies. The crew towing back after a record pass is something they cherish which is what photos are for. Panning shots are some of the most lost in the bunch, from the stands you can go to 20 on the shutter and track it for a few bursts, nail it and it looks like 500MPH, lol.
If by chance the track has no photographer on certain days, make yourself available and keep in mind they will like the idea as long as the official photographer isn't freaking out about it. Look to find access by shooting FOR a website with a letter of recommendation possibly; have the administrator call the track for access and so on. Once on the track and you produce, they will see the benefits of having you.
Hope some of this helps you on your way, this is from my own experience, We've published over 600 galleries, shot some of the most interesting races, shows, models and sports, yet I'm not in it for money or glamour, it's more about the racers and we had the outlet for them and it was appreciated by them.