Leopard seems to have a problem when you have more than two DNS servers set. As soon as I added a third DNS server to the list all connections nearly dropped out. It would take over five seconds to load any page.
The fix to this problem is to use only two DNS servers. I choose OpenDNS as their servers are reliable. I don’t like the ads they put on the pages when you type a domain wrong, but the benefits are good enough to look over this issue.
Are you using OpenDNS in Leopard itself or in your router? I use the OpenDNS servers in my router so that all of my computers can benefit from their reliable servers and also take advantage of their adult content filters.
I change my DNS provider about monthly. I normally prefer to have multiple DNS servers so if one dies you still have many more backups. Having 3 DNS servers is almost always the recommended setting for servers, just so you always have a working server.
The problem with Leopard is that if you have more than two DNS servers your entire network goes at a crawling pace. By limiting the servers to only two (opendns in my case) you go back to zippy speeds.
By setting the DNS servers on your router you can have as many as you would like (or it supports) as you will only have one on your local machine, the router.
I use OpenDNS as my ISP’s DNS, though the search is annoying, I find it great and I also like it’s little feature of correcting URLs.
I love OpenDNS myself, but I run into issues in my school network, so I don’t use it here at school.