An Airport Extreme operates over 802.11g (54mbps) and the newer 802.11n (108mbps) in dual-band mode (Allowing older g-clients and newer n-clients to operate simultaneously). 802.11n is faster, and operates over a greater distance.
An Airport Extreme has one USB port, to which you can attach a USB hub, to which you can attach several USB drives and/or USB printers. Computers on the network can access the attached drives and printers as shared devices.
You could plug your second Internet connection into the Airport Extreme, and connect your Airport Extreme to your existing wired network (Giving the Airport Extreme an IP on the same range as your existing wired network). If you do this, be sure to turn DHCP off on the Airport Extreme as two DHCP servers operating on the same network can cause issues. Once you've done this, your existing gateway device will hand out IP's and designate itself as the default gateway. You can then manually configure some of your devices (e.g. the servers) to have static IP's on the network range, using the Airport Extreme as the default gateway (As a cheap and nasty way to 'load balance' between your two Internet connections)
It's a bit of a convoluted and inefficient setup, but it will work. Be sure to thoroughly document your setup if you go down this route.
It's up to you whether an Airport Extreme would be a wise purchase for your situation. I can't speak for the quality or reliability of them, but the USB port could be handy if you can find a genuine use for it. There will be cheaper 802.11g/n Wireless Routers without the USB port, and there may be cheaper ones with the USB port, I'm sure someone can comment on this.