I think this conversation is absolutely on point, as I am sure a NUMBER of people will want to know if the D600 can perform well in tough conditions in low light.
Clearly, I do not have experience in what you are doing. What I do know is that shooting in dark civic centers and indoor arenas with my D600 at ISO 6400 or below is giving me results VERY nearly equal to my D3s. It does take a bit more time to get focus, but then I am dealing with human sized targets and not horse sized targets.
If you've got enough glass to make that horse take up a large portion of the frame (1/3 or more) I'd say you'll do just fine. Once you're trying to track an object that takes a relatively small part of the frame things get more dicey.
So here's what I suggest. Buy the D600. Save all receipts, etc. Shoot it with your current gear. Find out if it will work for you by using it on a non-paid shoot. If you don't like it, you can sell it. You'll lose maybe $150 on the deal. Essentially the cost of a 3 day rental. That's pretty tough to beat.
>>It's not the VR. It's that the 70-200 has it's own >focusing >>motor built in unlike the 80-200 and it focuses literally >>twice as fast. HUGE difference. I never use the VR on >this >>lens. > >Understood...though at the time, since I didn't need VR and >since cutting horses only move side-to-side on the same focal >place, the 80-200 made the most sense. The 70-200 will be a >ways down the road. > >Let's assume, for a moment, that I did have the 70-200, like >you do. Is it your opinion that the D600 would be able to >keep up with the action as I described above? As you said, >during the day, shouldn't be a problem since my D200 can do >it, but if I get a camera that allows be to up my >"game" to a covered arena, then knowing whether or >not it will grab a dark brown horse, moving quickly, as well >as a D3s (or even a D3) is something I'm trying to determine. > > >Here's where all this winds up: I'm trying to market myself as >a second-shooter to other photographers, so being able to >shoot things like concerts, weddings, corporate events and >such are things my new camera needs to be able to do. However, >my current paying gigs are for rodeo and horse show events, >mostly outside and mostly during the day...but that's been >because the D200's ISO performance hasn't allowed me to shoot >anything else. > >Were I actually employed in a full-time corporate job (rather >than being unemployed for as long as I've been), I'd probably >end up with both the D600 and D3s. But I'm not, and the money >I've saved toward my new camera needs to get me something that >does all of these things well...if that's possible. > >I'd love to be able to save money by just getting the D600, >but since my original plan was a D3s, I'm prepared to get one >of those if it's the better of the options. Where I see the >D600 being equal to or "better" than the D3s is the >image quality, the amount of data to work with (cropping, >etc.) and low-light performance. Where the D3s seems to excel >is the super-high ISO performance, burst rate and, probably, >overall build quality. > >If this is a discussion better taken off-line, let me know as >i don't want to bog down a discussion any more than I already >have. > >Thanks, >Tim