Welcome to the discussions, araym!
If I get an AE, can I set up the dual bands such that: a) each one has a different SSID, b) I can force my iPad Wi-Fi + 3G to connect to the 5 GHz band at 802.11n, but have my older iPod touch 2G and older Macbook to the 2.4 GHz band at 802.11b/g speeds? And if so is that how ya'll would recommend that I set this up?
If you get the dual band AirPort Extreme, by default both the 2.4 GHz and 5 GHz bands are set to use the same network name (SSID). You can change this by using AirPort Utility > Manual Setup > Wireless Tab to set up a separate name for the 5 GHz and 2.4 GHz bands.
Since the 5 GHz band will have a different name (SSID), if you set it up this way, you can point compatible devices to that network to use that band. Likewise, older "b/g" devices will log on to the 2.4 GHz band.
And if so is that how ya'll would recommend that I set this up?
If you want to "force" a device to use a given band, the info above about setting up separate SSIDs would be the only way to accomplish that goal. Otherwise, with both bands using the same SSID, devices will connect to the band with the strongest signal. Depending on their location, compatible devices may or may not connect to the 5 GHz band if they see a stronger signal from the 2.4 GHz band.
Likewise if I get the Airport (not Extreme) to save a little money, am I correct in assuming that even though the router will support 802.11n I'll be forced to the lowest common denominator because of the two older b/g devices on the WLAN coupled with the fact that it is single band (one at a time)?
I think you may be asking about the AirPort Express here, which is single band device capable of both 5 GHz and 2.4 GHz setup, but not both at the same time. You would need to use the 2.4 GHz network and a compatible Radio Mode setting like 802.11n (802.11b/g compatible) so that all of your devices could connect.
Faster "n" devices would connect at "n" speeds and older "b/g" devices would connect at the slower speeds. If you have both "n" and "b/g" devices active on the network at the same time, the slower devices will tend to pull down the speed of the faster devices, but they will usually not drop all the way down to "g" levels unless there is a lot of activity (like a heavy download) occurring at the time.
One final note and that is that you should understand that although 5 GHz signals travel faster, they are not as strong as 2.4 GHz signals. Therefore, 5 GHz does not penetrate walls or other obstructions as well as 2.4 GHz. Many users and are finding that they almost need a line-of-sight situation between the computer and router for 5 GHz to work really well. In other words, forcing a device to use 5 GHz does not necessarily mean that it will perform better than it would at 2.4 GHz unless it has a really good signal from the 5 GHz band.