I ran into problems with this before... I was using a Dell PowerConnect switch with a Cisco switch. Apparently the Dell's are a little picky in what they connect to. Try putting in a different switch if you can.. or try using the switch instead of the router (manually set IP on computer) to see if you can still get through.
I'm going to say the PowerConnect doesn't like talking to your new Dlink.
Did you check the Dell site for possible problems for this switch?
I gather you're trying to get Gb transfer between your NAS and PC's?
Let me get this right: You have a NAS connected to the switch. Also, a cable connects to the router. When you remove the switch and connect the NAS to the router, which is connected to the PC/laptop then your able to access the NAS and get internet? If this is true, like Riser stated, there may be a mismatch between the switch and the router.
One other silly question: Since the Dlink is a Gb router and your NAS has a Gb NIC why don't you just connect it to the router? It has 4 ports. If you need to connect more PCs then connect them through the switch.
I have several PC's in my home network. To get them connected I purchased some GB switches and hung them off the supplied router. I get GB through put to those PC that have GB NICs and I hung the slower PC's (that have 100MB) NICs on another switch.
I guess I'm missing the point of having the switch before the router. Why can't you connect the Dlink router directly to the firewall and then connect the switch after the router. You would be able to Gb through put to all attached nodes? Am I missing something?
The D-Link Xtreme N™ Gigabit Router (DIR-655) is a draft 802.11n compliant device that delivers up to 14x faster speeds* and 6x farther range* than 802.11g while staying backward compatible with 802.11g and 802.11b devices. Connect the Xtreme N Gigabit Router to a cable or DSL modem and provide high-speed Internet access to multiple computers, game consoles, and media players. Create a secure wireless network to share photos, files, music, videos, printers, and network storage. Powered by Xtreme N™ technology and equipped with three external antennas, this router provides superior wireless coverage for larger homes and offices, or for users running bandwidth-intensive applications. The DIR-655 also includes a 4-port 10/100/1000 Gigabit switch that connects Gigabit wired devices for enjoying lag-free network gaming and faster file transfers.
If so it's a Gb router. Maybe I just missed something ...;>)
By having the Dell switch to expand the Gb connection of your network would eliminate the Zywall bottle neck.