There is mixed opinions on Topsites. I find they work well and provide good results and they do go well with directories (they are a both a similar concept). It will bring people into your site aswell. People will place the topsite banner on their site and that is a link into your site and people will click it and can move on to view your directory when if it wasn't for your topsite, they probably would not have visited your directory.
One thing you need to watch out for are the spammers. It will take a while to build a good blacklist but then you won't get many.
I don't see why you shouldn't, David. As Fowler said, topsites usually do work well with directories. Just make sure the topsite relatively has the same look as the directory. I mean, you don't want them to look like two completely different sites.
