The router alone can't do it...
What you're trying to do is commonly referred to as "captive portal" to redirect users on the network to your webpage.
There is a page on DD-WRT (an alternative firmware like Tomato) that implies you can do it: »www.dd-wrt.com/wiki/index.php/Captive_Portal
...but I don't like being dependent on a 3rd party service - and I'm not about to start coding someone else's sourceforge project that they'll abandon next week.
If you want to spend all day building something from scratch on Linux look here: »en.wikipedia.org/wiki/Captive_portal
If you want something that runs on a Windows I'd suggest »dnsredirector.com
and you can be up and running in 30min.
The suggestion to separate your internal LAN (wired clients) from the wireless LAN (guest's) using VLANs is spot-on... for security reasons... you don't want foreign guest computers to come with viruses or other nonsense and then get into your internal network - do you?