Well to answer your first question about the mobo's: Yes there is a benefit to getting a higher priced/better board, but is it worth it to you?
With a better board (such as the new DFI, or MSI Neo, or Epox) you may be able to achieve that 2500+ OC easier and more stable. This is primarily because the Vcore voltage on those boards will allow you to go higher. Also the Vdimm will allow (correct me if im wrong) a 3.3v increase.
My board, the Gigabyte K8NS Pro 250, which im extremely happy with, limits my vcore to 1.7 and my vdimm to +.2v (up to 2.9). Alot of this is a matter of preference. I can get over 2400 with my 64 3000+ CH. Although the NC's OC better, the CH will give you that added cache that the NC does not have, so its a trade off depending on what you are going for. As far as the Mhz range of the OC between the 3000+ and 3200+ you can check out the sticky in the AMD section about peoples benchmarks they got. As with anything, you know that cooling makes all the difference, and so does your fsb/ram ratio. If you get some ddr500 memory, you can use your 1:1 ratio and get better performance.