Well for ease of updating the website I'd suggest something like joomla. It's a good base for your website, you can edit content easily (a bit like using word). There are many plugins to do whatever you want, whether it's a gallery, email forms, shopping cart to sell your photos etc.
It's free, which is always a plus side.
I'm not sure about the US hosts apart from Siteground (which I have some experience of).
From an SEO and load time a HTML site is better than flash. Flash depends highly on the speed of the persons internet connection and their computer, some sites I've seen take quite a long time to load. Also I much prefer HTML to Flash as it works across many areas, for example mobile phones have the ability to read HTML but lots cannot and will never read flash.
As thecackster said, you need to keep your images down in size. I usually use level 8/12 in photoshop for saving photos and never have any above 600px.
For design, again joomla has loads of templates out there and it's pretty easy to install and edit them.
Hosting your photos that are for sale should be on your website although it's always a good idea to have a flickr account, facebook and others. The more people find you the better.
PM me if you want help with your site.