@Mix3d Nuts
I have it set to full Mixed mode (not only G and N) and there should be little or no performance impact on the N clients. In Mixed mode G clients will work at G speeds and N clients at N speeds, however, if you use them at the same time, they will obviously share the wireless channel's bandwidth.
So if you have a G client connected and active, it will use up bandwidth and you won't be able to achieve full N rates on the N client until the G one goes idle. But theoretically, yes, the G and N clients work simultaneously at their supposed rates.
Another thing to consider is the wireless security. This might sound strange, but unless you don't use WPA2 authentication with AES encryption, you will not achieve N rates. It's part of the specification. If you use something lower, your N client will fall back to G rates.
Also, the N specification is capable of 40 Mhz bandwidth, and by default DD-WRT and other firmwares set it to 20 Mhz for the 2.4 Ghz band. You can read more about why here: http://www.dd-wrt.com/wiki/index.php/Wireless-N_Configuration . I turned mine to 40 Mhz though and it works fine.
Channel selection is also important, if you are in a densely populated area with a lot of wireless networks around that use the same channel as yours, the performance will be poor. Try experimenting with different channels to see if there's any difference. Also check the rate on your Status > Wireless page.