Routing 1 LAN + 2 WLANs + 2 WANs (failover) with DD-WRT on WRT54G2

I've installed DD-WRT v24-SP2 micro on a Linksys WRT54G2 v1.0, and I'd appreciate some help on setting up a not-so-trivial network.

I've studied the documentation (notably http://www.dd-wrt.com/wiki/index.php/Default_Configuration_Overview, which describes the router's ports), but I can't figure out by myself how to achieve what I want. I have some knowledge on Linux networking and routing, but nothing too fancy.

Description of the networks:

I want to set up one wired LAN (maybe someday I will partition this into two and connect them with another router, maybe this information could change the way to configure the WRT54G2 now, I don't know).

I want to have 2 WLANs, one internal and another for guests.

The internal one should be completely connected to the LAN (that is, all boxes on the wired network and the internal wireless network must see each other).

The guest WLAN cannot see the LAN, nor the other WLAN (the internal one).

All the computers connected to any of these networks (LAN, internal WLAN and guest WLAN) should be able to obtain IP addresses and DNS server addresses from a DHCP server on the router.

Internet connectivity (dual WAN with failover):

Finally, I have 2 internet connections: a fast one through PPPoE and another, slower on, to which I must connect with DHCP (to obtain a public IP address from the ISP through the cable modem).

All the networks (LAN, internal WLAN and guest WLAN) must be able to access the Internet.

The two connections should work as follows: the faster (PPPoE) one is the "default", every computer should use that connection unless it is down (this may be detected by pinging the gateway, or by any other means if anyone has a better idea), in which case the router should automatically switch to the slower (DHCP) connection. If the PPPoE connection ever is available again, the router should switch back to it once it detects that the connection is available.

Can all this be done with a single DD-WRT micro running on a Linksys WRT54G2 v1.0? What are the VLANs needed, what bridges should I create?

I'm pretty sure I'd be able to setup these networks using a Linux box with 3 network cards (one for PPPoE, one for the other internet connection, and the third for the LAN) and two wireless routers... but after reading the description of all the hardware that can be controlled by DD-WRT, I have the impression that it can handle all of this!!

Thanks in advance!