If your web pages do not contain any ASP code, making it .HTML is best. Are you running forum software or some free script software? Some scripts use a lot of CPU cycles. Web hosts do not like customers to use these scripts because they slow down service for other customers on that shared web server.
For the past 2 months I made several changes in the URL's of one of my sites. At the begining I used page.php?c= Then i started using mod_rewrite and the URL' changed completely. Two weeks later I had to change the ID's at all of the pages, so the URL's were again heavily changed... None of the changes affected the performace of this site on the SERP's. Google just replaced the old URL's with the new ones on next update. Unlike what you might think looking at the mess on the SERPs now, Google IS smart and it's easy for it to tell that the pages with the new URL are the same pages. There are lot's of ways to identifya page. Durring all of the changes I did not changed anything in the head section of the HTML code so I can prove to G that I'm not cheating :-).
That's the same as G handles the www. hosts and mirror sites. I have another site that is accessible via 4 hosts - site.com, www.site.com, aaa.othersite.com, bbb.othersite.com. Every page is accessible via any of those addresses, but when G serves a page on a SERP, it chooses one of the 4 addresses.