The New Recommendation(s)
The bottom line is that I'm no longer recommending that you go for dual-band dual-radio products if you want to move now to draft 11n. The main reason is that the potential upside isn't enough to justify the high cost, especially if you have a large installed base of 802.11b/g devices. As I showed in this article, you're likely to create more problems than you'll solve by trying to run a mix of 11b/g and draft 11n devices on one router.
In fact, if you have an installed base of 802.11b/g clients and devices, I'd recommend that you just buy an 802.11g router if the one you have gives up the ghost on you. They're cheap, the selection is wide and you won't have the compatibility hassles that introducing a draft 11n router can bring.
If you want to try draft 11n, try an entry-level two-antenna model. I liked the D-Link DIR-625 in our recent round-up of entry-level draft 11n routers, but there are also other good choices in the group. An inexpensive draft 11n router will give you most of the performance gain that there is to be gotten from draft 11n.
Because you won't have dropped a bundle on your first draft 11n router, you won't much care about upgrading it when the standard is released in 2009. By then, dual-band products will have dropped in price (both single and dual radio), so you'll be able to buy a "finished" (not that these things ever are) product. Much better than waiting for your manufacturer to squeeze out an upgrade.
If you like your current router, but just want to see if draft 11n wireless can provide a benefit, go ahead and buy a draft 11n router and just turn it into an access point using this simple how-to. And definitely run both the old 11g and new draft 11n WLANs side by side if you have a mix of "legacy" and draft 11n clients.
If you really need to move to 5 GHz, then buy a single-radio model, unless prices drop so that the dual-radio price premium isn't so high. As I was writing this, it looks like Linksys might be moving in that direction, with a $160 street price for its dual-radio WRT600N. And for those of you pining for the D-Link DIR-855, buy something else and get on with life!