Steps to finding a good website design agency or freelance web designer
It is usually a good idea to use a design company that is based locally to yourself, for example if your company is based in London, then it may be a good idea to use a London web design firm or web designers in Essex. This will allow you to visit their design studio and speak to the developers in person and get a custom built website.
Cost - make sure you are not paying over the odds and get a complete cost for both the design and development of the site. If you want a cheap web designer for a start up company, freelance designers will usually work at a low cost as they will have little to no outgoing costs, whereas a larger firm should look a company that can create e-commerce, CMS, and dynamic sites.
The design - It is common knowledge that a visitor to your site will click away if they find the design unappealing. The best way to choose a web design agency is to look at the design of their site, and if it's appealing then look at their design portfolio.
SEO - I would advise not using a company that just specialises in SEO, unless your website has already been built. Instead use a website design company that will look at optimizing your site when building it. There are plenty of SEO web design firms in the directory that can meet your needs. The reason being is that a lot of the requirements of good search engine optimization need to be applied during the initial build, such as the use of correct keywords, META tags and a good use of clean HTML code. Any designer offering a adobe flash site will not be offering good SEO. Other forms of SEO, such as link building or URL exchange can be implemented afterwards. Gaining a high position in Google, Yahoo or MSN may take some time if you are entering a competitive search phrase or term that has over 10 million other results. It is advisable to ask the SEO company to focus on more specific and smaller searches rather than one big one.
CMS (content management system) - many web designers will be able to give you a website that you can maintain yourself will no knowledge of website coding, that should be easy to use. If a web design company does not offer you this service, I recommend looking at PageLime, who offer a really useful remote content management system.
I hope this information has been useful and helps you find a good web development service in the UK or USA.