First of all, the current rating on the power supply is a maximum current the psu can deliver, so if you have a 5V / 700mA and a 5V / 2A supply, then a single 5V / 2.7A supply will do the job of both of them.
So, what you need to figure out is what voltages you really need and sum up the current for each of them.
The voltages you need might not even be all the once you have now.
Lots of devices will be able to work fine on a much higher voltage than the supplied PSU, I have several network switches that come with 9V supplies, but they are built for and works perfectly on everything from 6V to 24V.
That Arduino will be happy with 12V, though the regulator might get a little hot if you are loading it down a lot.
The only way to know for sure which voltage a device will work with is to open it up and look at the regulator section of the device.
Your 5V devices will probably not be happy with 12V, perhaps a 12V powered USB hub would be a good idea there.
All my on-always gear is powered by a single 12V / 5A power brick, which is much more efficient and compact than the 8 separate wallwarts that the devices came with originally.
I made a small box with a load of 5.5mm barrel DC connectors, then I bought a good hand full of DC connectors that fit them and then I either cut the original power cables and fitted my new DC connectors or I simply made all new power cables for things.