http://blog.woodboilers.com/2009/06/properly-sizing-pex-pipe-for-remote.html
Very nice and practical article for selecting pipe/pump combinations that will work. They are doing their customers a good service. It will make it easy for new installers to select from a table a combination that has allready passed the formulas and keep fluid dynamics for another day. Possibly never ...
http://flatplateselect.com/site/pub/login.aspx?ReturnUrl=/site/Default.aspx
here is another that I cant say enough about. Understanding the impact of liquid material, flow rates, BTU loads, temperature differentials would have you pulling out hair. Use this tool to find and size the HX and flow rates you will need to get the heat out of your boiler and into your tank. Make doing what if I ... variations simple. I wish I had bought my HX from them to say thanks. ( You have to sign-up/login , it's free and what is on the other side is well worth the trouble )
http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm
This has disclaimers about accuracy but it was dead on for my home and allows a DIYer to cut the crap and get a working number for the BTU load. I cross checked the results by comparing the annual estimates with the Gal of propane I actually purchased. Scary close almost like it is not magic just basic science.

I have a year now of watching tank loss and outside temps to say the number this site allowed me to get ( Without fancy software or Hiring a manual J calculation ) was right on.
For a person looking to get started these 3 are gold. They answer some of the big starter questions, how much heat do I need, What size: boiler/tank/pipes/pumps/HX and then you can start to get an accurate cost in mind.