Hi everyone I'm looking for some advice.
After having my HD broken I decided to go with a NAS mirroring.
I will have soon a D-Link 323
As it support Gigabit Ethernet I will take this to make some upgrade to my lan too.
At the moment I have a Netgear DG814 Modem/Router that I use for the lan too.
I was thinking to buy the D-Link Gigabit Router 4300/4600.
Now the question is, should I but a Modem too or I can use The Netgear only as Modem disabling all the router function?
And (noobest question ever) are the cable for lan 100 the same of Gigabit or I need new cable?
You shouldn't need to replace the router -- just a GbE switch should be fine, and should the DNS-323 ever support or get support hacked in for jumbo frames, perhaps give a further performance improvement.
A Netgear GS105/GS108 for example would be less expensive, perform well, and support jumbo frames should you get pairs of devices which also support it. Note that you use up one port linking the router.
Cat 5 is good enough in theory, but (factory terminated and tested) cat 5e would be recommended for new installations. You could just try it and see if a replacement is necessary due to bad links / link speeds.
If you really need a router, and don't insist on jumbo frame support the DGL-4300 is a good choice. It can use DSL modems as PPPoE. You'd need to check the details on your router/modem to see if it can be used just as a modem -- aka as "bridged". The DGL-4300 doesn't support PPPoA AFAIK, so if you need that you'll should look for another model.
The DIR-655 might be an interesting option, but not that much is generally known about it because it's not as commonly used as the DGL-4300. Draft 802.11n is also a shifting target.
As the router will only have to deal with LAN=>WAN traffic it wont need to be gigabit as your WAN link will be teh bottle neck, not the 100mbit port on the LAN side, just buy your self an X port gigabit switch - x depends on how many pcs you have + 1 - as you need one port to go to the router.
For the switch depending on what you need you could go for an unmanaged switch, or get a managed one (costs more) that is non blocking and supports jumbo frames.