I think they only pay attention to the anchor text of the first link, this would stop people from stuffing multiple links to the same page each targeting a different term. So if you have a few text links to the same page make sure the first one contains your primary key phrase.

Nofollowing the rest of the links wouldn't have any positive effect. The total link juice a page can pass on is divided by the total number of links on the page (regardless of follow or nofollow). In fact it can have a slightly negative effect as the juice from the nofollow links essentially evaporates instead of being passed to other pages on your site. Here is an article on how this works.

http://www.mattcutts.com/blog/pagerank-sculpting/

I've recently undertaken the same task and I decided to make a decision based on the quality of the directory. If you currently have a low amount of back links with your brand/domain in them then changing the anchor text on some of the links will definitely help you build a more natural and strongly branded profile.

Install a page rank toolbar if you don't have one already and check the PR on the home page of each directory. If it comes up as 0 then rather just remove the link (if possible) as it's either a low quality directory to begin with or it's been totally devalued by Google.

I would try and stay clear of doing this to be honest. It won't validate, older browsers may have problems with it and I'm not sure how search engines would treat it (they may not follow it). There are better ways to do it.

If you set your anchor tag to 'display:block' you can then set the height and width too. This way the whole div would be the link, not just the text.