I'll probably put this info up on a personal blog shortly.
Step 1: Shop for a Dual-Band Wireless Router
If you took the shortcut, you can skip to the next step. Otherwise, here are more details in case you are obsessive about saving a few dollars at the expense of a bunch of your time, kind of like me: Find the cheapest dual-band router that is listed as supported by dd-wrt. I found a pretty good deal at the above link. CNET Reviews also points to CDI as having it for $77. If I wanted to spend over twice as much on hardware and run a testing release of dd-wrt, I'd get the Linksys WRT610N (~$165) since it is supposed to support *simultaneous* 2.4 and 5 Ghz operation, but who knows what is actually happening with both radios with dd-wrt on this hardware -- maybe it's the same. This Netgear also doesn't have gigabit ethernet ports like the Linksys, but you could hang a gigabit switch off of it for $50 or less.
(Added June 12, 2009: Buy.com now has the Trendnet TEW672-GR 300 Mbps dual band wireless router for $67 including shipping. I don't yet know if dd-wrt will work on it, but I'll probably get one and try it out, and update this instructable or start a new one.)
Here is the list of dd-wrt supported hardware, and here is the router "database" where you can type in a few characters of the router's name.
It would be better to use and support open-wrt because of repeated GPL violations by the dd-wrt guy, but I was more certain that this would work with the hardware I was buying, with a comprehensible UI, dual-band, and I need it to work unattended because it's going up a mountain to my Dad's house in North Idaho, a thousand miles from where I live. If the X-wrt or OpenWrt guys come up with something that I can easily figure out, I'll happily switch to one of those. From the main page, I couldn't even tell if those run wifi (they do), let alone support my hardware, and dual-band radios. What I gleaned from the open-wrt web pages are that they're all general-purpose embedded firmware or some such. Abstraction is great, but come on, what does it do? </rant>