AFAIK yes XP has a 2-GPU limit, so a 4850X2 is all you get because it counts as 2 GPUs. On Vista, it should work, and they don't even have to be from the same manufacturer, I think.
I think I'd prefer a single HD 4870X2 to a setup with HD 4850X2 + HD 4850. I suspect it will cost about the same, two GPUs are better supported than 3, and the drivers for a single HD 4870X2 have already been tested by plenty of people. Wait for benchmarks if you can...
BTW, if you're planning to buy a HD 4850 now, you may want to know that you can also combine it with a HD 4870. The result is better than HD 4850 Crossfire, there's no dumbing down.