Using digg, reddit and other similar sites is only useful if you are oriented on hot trends or so.. then there can be really fast good traffic made, but will it have influence on sales also depends on many things..
It's better to submit to StumbleUpon i usually get 100 visits a day and sometimes up to 1000 visits a day from stumble. Digg usually can only send 5-10 visits a day if that. Reddit is good if you know how to upvote your own post using ipchangers and multiply accounts to around 100-150 upvotes then you will receive around 8000-20,000 visits from that one post. I suggest submitting to stumble and reddit for best results.