I think that you can see a compelling argument for keeping your existing router when you decide to move up to draft 11n. And even if you're upgrading because your old router has finally died, you should pick up another 11g router to replace it, unless you plan to move all of your wireless clients and devices (such as media players, Wi-Fi Skype phones, etc.) to draft 11n.
To set up your two routers, you'll need to change one of the two routers to act as an access point, so that you don't end up with clients on two different subnets. Just disable the DHCP server of one of the routers and set its LAN IP to an unused IP outside the range of the other router's DHCP server. If you need more info, just hit this article.
Of course, you will need to set the APs to different channels, using only 1, 6 and 11, make sure the draft 11n router is using the 11g friendly 20 MHz bandwidth mode and use different SSID's for each router so that you can tell them apart.
It's also probably a good idea to not sit the two routers right next to each other, in order to minimize possible interaction between them. You might even try using a pair of HomePlug AV adapters to really get the most out of having a second AP by locating it in one of your home's dead spots.
But no matter how you do it, setting up a separate 11g wireless network when you bring a draft 11n WLAN online should help make the transition a smooth and satisfying experience.