From August 2014, Google will be increasing the rankings of sites that use SSLs, so a lot of clients will be wanting to know what they would need to do to exploit this.
Simply put, any SSL we provide, even the most basic offering will be sufficient. From a technical standpoint, Google say they require 2048 bit keys for the certificates, which is what we use, but might not necessarily be what a third party SSL uses.
Other than simply installing an SSL, they will also need to actually run their site through it. The easiest way is to put a redirect to force traffic over SSL in the .htaccess, which you can find in the KB.
Whilst google haven't explicitly said this, it's likely that a site that uses an SSL but loads insecure elements will have a negative impact, so it's best if they use relative URLs rather than hardcoded ones (ie. in the html have an image path set to images/foo.jpg rather than http://website.com/images/foo.jpg). If a site has problems with loading insecure elements (so you see the yellow padlock in your browser) you can use Why no padlock to see what's being loaded insecurely.