What to buy?

108 Mbps is actually pretty good. There aren't really many that are higher speed. DSL only gives you at best 8 Mbps for the internet. So any slowdown you have is within the network of devices that are hooked up. You could see if your router needs a better configuration by contacting Netgear. 108 Mbps is going to be slowed down by your slowest wireless device on the network that uses 802.11 protocols. Not all phones do.

As long as all your devices have 802.11n support, the Airport Express should suffice as the soll wireless transmitter. If any do not have 802.11n support, connect them to the ethernet ports of the Netgear. Find out from Netgear how to disable the wireless on it, since it is limited to 802.11g. The modem part of it may need to be provisioned by your internet provider for it to work as such.

as brody said: Your Netgear router is only 802.11g (half the speed of 802.11n).

Your equipment must be "n" too, to use the max speed of a 100mb/s router.

Higher speed by connecting with the ethernet connector, there is one ethernet connector on the Airport Express, and there are 4 on the Airoprt Extreme.

So connect those two by ethernet cable to the new Airport Extreme Router (which has 4 Ethernet ports).

The interesting thing is: with the Extreme you can set two simultaneous bands (2.4GHz and 5GHz) and certainly a number of your devices have the possibility to connect to the 5GHz band, which gives a much higher speed: use Network Utility to see (in Mac/Applications/Utilities), it will run at linkspeed about 300-500Mbs.

You are right, 1Wan 3Lan 1USB.

The Extreme is downwards compatible: it accepts connections from g and n devices at the same time. And if you have the 5GHz band set, it will accept n 5GHz and n 2.4GHz and g 2.4GHz connections at the same time. Waiting for the new "ac" routers: not necessary, none of your existing devices have the "ac" possibility.