I created a web-site for a client with W3C Valid XHTML and CSS Code, Semantic and Accessible Mark-up and I had said to client my code will be SEO friendly. Theyway i code it will be good for your site SEO. I putted my all efforts to make good code
Now my non-techie Client is asking me, Should him go for any SEO company even after providing SEO friendly site by me?
What other SEO companies will do for him other than what we can't by W3C Valid XHTML, CSS , Semantic and Accessible Mark-up?
That depends on the SEO needs. In general you don't have to do much at all, the search engines will still be able to find some information on the pages. If you need the site to do really well in search engines, SEO frendliness is not enough, but it's a good start.
SEO is more than just making the information accessible, it's also about putting the right information in the right place. For example making the page title and headline correspond, and finding the right information to put in them to make search engines find them for the searches that you want.
Valid HTML/CSS has very little, if any, bearing on SEO. Neither does a lot of semantic or accessible code either, but I'm guessing by that you mean you've use heading tags and the <title> tag where appropriate, used ordinary links with <a> tags (instead of behind Javascript/Flash). In which case, the on-page SEO is probably quite good.
But a large proportion of SEO comes from 'off-page' factors like how many links point to the site. An SEO expert would probably look at gaining as many in-links as possible, adding the site to web directories and trying to promote the site in various other ways (perhaps through Facebook/Twitter campaigns).
If your background isn't in marketing then you may not be able to do this very well and hiring an expert would be a good option.