First of all I highly doubt the load on your router is a problem for it. Modern routers can handle far more load than the internet connection usually allows, so that is where your bottleneck probably lies.
If plugging in more devices to your router is needed, then just get what is called a switch... like this
... they are essentially like power boards for networks. Just plug it into port on your router and your essentially has more ports on it.
If you want to extend the wireless network into another part of the house, then an extender or a basic access point may be a better bet. Don't worry, you haven't done your money.. the N900 will do the job fine but you need to disable the routing functions.
Power line networks are a good option for hard-wiring your N900 in another part of the house but the trick here is you want one end of the power line connector plugged into your existing router and the other side plugged into one of the 4 Ethernet ports.... not the internet modem port. If it is plugged into the internet modem port, I can see a good technical reason for bad performance.
If you don't care about the technical details skip to next paragraph but I ill try and explain. When your modem connects to Testra it gets an IP address that is visible on the internet. When you plug a device into the modem, your modem gives your devices a private IP address which is only valid within your house. To connect to the outside world your modem then translates the private address to a public one and visa vesa. Your new router will also be doing the same thing, so for any devices connected to it are getting this translation done twice... a well known performance killer in networking circles.
So what I would do...
1. Connect a laptop to the new Netgear and open its configuration options... the manual should help you there.
2. Look through the settings until you see an option called DHCP Server and disable it (you don't want it allocating IP addresses on your network, your existing router already does this)
3. Unplug the modem port on the Netgear from your powerline adaptor and plug into one of the 4 Ethernet ports
4. Turn your laptop off and back on again
What you will have here is 2 wireless networks, likely with different names, so just connect to which ever one has the best signal for where you are. There are ways of setting up one network that roams between access points but not sure the Netgear supports that.
Edit... if you do the above and something isn't right, there is a chance that you will break the whole network and no devices will connect to the network... even on the other end of the power line network. If this happens, don't panic, just disconnect the Netgear and switch off and back on anything that isn't connecting properly and it will be good again. It probably means your Netgear is still handing out private IP addresses and you need to try again to disable the DHCP server function. Also don't worry about mucking up the settings on the Netgear, your manual has instructions to reset it back to factory default settings so you can start again.
This post has been edited by TheWanderer: 15/02/2013, 07:18 AM