Contrary to rumours Google does not penalise 404s. There is however detremental impact on the ranking your old pages had attained, the associated keyword matches for your old articles and of course user experience.
It's possible for you to relink those pages back to new pages on your new site and carry forward, as it were, the ranking/reputation etc. of your old content (Although leaving it for 6 months is probably going to have negated this action).
Using htaccess is one way, you could also use an extension to help. The core extension called Redirect is OK for small number of links but in the best tradition of Joomla it offers a cumbursome interface.
Some extensions like SH404SEF will try to match urls and redirect to existing content as best it can.
My preferrence is to use an extension called ReDJ. This allows you to create rules for dynamically rebuilding urls from old ones to redirect to new content.
It is extremely powerful and does have something of a steep learning curve but the developer is active and very supportive.
It also tracks urls and how many times they have been visitied and provides an interface to create direct 1:1 redirects.