well i think ders 2 type of licenses for SimpleDNS, one of limits the number of domains to 25 or sumtin, the other gives u unlimited.
It doesnt matter if u use SimpleDNS to run your own webserver or jus speed up ur browsing by runnin ur own DNS.
Now as of which OS to use, I cannot say one is better than the other. Apache n PHP on linux has undergone through tests and is said to be more stable than in windows. And running a linux box will considerably reduce memory load cuz it doesnt have many resource consuming services in the background. But if ur not familiar with the Linux world, u mite find it REALLY annoying with out a proper GUI. To tell u the truth i started learnin Linuz last month. I dual boot it with XP.
So to summarize everything, if ur familar with Linux, and is planning to host a proffessional site, i recooment a Linux system. But if u like things to b easy n quick i recommend WIndows. I use XP for local development, since i dun have a second box n im not hosting ne proffessional sites.