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I would avoid DD-WRT if I were you. The DDWRT development community doesn't seem to perform any real testing on their releases and I have yet to find a version that does not have significant bugs.
I would recommend you try another open source firmware such as Tomato or OpenWRT, particularly if stability is a concern. You will save yourself hours of headaches.
It doesn't hurt to zap an older 802.11g router and replace the firmware with DD-WRT, just make sure you have a way to reflash it back if you have trouble with DD-WRT. I loaded it on my Linksys WRT-54g and was quite impressed by all the options: new settings for DNS, DHCP, security, SSID aliasing - pretty amazing to see how many more features could be added to the most stable wifi router I have ever used.
There are some limitations to Wifi-N - the frequencies it uses seem to not go very well through building structures (walls, floors, ceilings), so be careful if replacing your Wifi-G to make certain that you can actually get a signal where you want a signal. If you have a bunch of offices to upgrade, you may want to purchase the new router and test it thoroughly before rolling it out in all the offices.