Wifi Question

[FireFox]

So I'm looking at expanding & upgrading my Wifi setup at home & have ran into some Questions.

1) I have Mostly wireless G & 1 wireless N device.
2) I have some deadspots in my wifi Setup I'd like to eliminate.

Now I've done Some research but not much & am a bit confused by some of what I've read so far. As I have both G and N devices that will be Connected at the same time I've heard some say that the wifi will be dropped to G speed but someone else said if you get a dual band Router that can simultaneously work in 2.4 a 5 you can run both at their max but another site said No. And to my knowledge 2.4 & 5 have nothing to do of running full G / N Speeds as N also runs in 2.4. So the Question is should I look at a dual band Router or a Single 2.4 Ghz Router.

Next in my search for a replacement Router I found that My current router is infact a modem & router combined & all the units I've been looking at is only Routers. Will it be possible to Connect my current rourter to run only as a modem? I was thinking down the line of disabling the DHCP server on it & then hooking up one Ian cable to it & plug it in to the wan port of the new router & let it do the routing function & then run a Ian cable from it to my Gigabit switch for the rest of the wired network.

These are my immediate Questions but there will undoubtedly be more as I continue my research which l will ask then.

Thanks in advance.

[Edit] Okay I think I got some of the above a bit wrong. After Some more reading I saw that Wireless N can also run at 5Ghz & you can do both G & N at max speeds at the same time if you set N to 5ghz on a dual band Router. The catch is that your wireless N device also needs to be able to do 5 Ghz. Further digging got me to my N device Specs that is as follows.From this it would seem that my N device can only run in 2.4 Ghz & not in 5 Ghz which mean getting a dual band Router wont do much as it will only operate at 2.4 Ghz & thus the speed of the N device will be dropped to G speeds if a G device is connected. So if I'm not planning on getting any N devices that works at 5ghz getting a dual band router is a waste of money & I'd be better of gelting a desent single band N Router as they claim that their range is better than normal G to try & eliminate my deadspots at least?