You should be able to get and install either the Intel (proset) or Broadcom server utility - depending on which kind of NIC you have in your server.
Once you have that on your machine, you can configure fault tolerance on the server. At least with proset (don't have a broadcom server w/ the tool installed right now to look at) you would configure a new team, tell it what type of fault tolerance you want - Adapter, switch, load balancing, etc.
The only gotcha I can think of right now is if you are running MSCS on any of the servers they don't support NICs setup with fault tolerance in any way and the cluster verification will fail.
Some keywords for you to look for "Adapter Fault Tolerance" "NIC Fault Tolerance" "Adapter Load Balancing" "Adapter Fail over"
I think a picture says 1000 words. So you basically can have three situations.
1 - Switch redundancy, but no Router or ISP redundancy:
router spof http://www.brokenhaze.com/sf_images/router-spof.png
2 - Switch, and router redundancy, but no ISP redundancy:
isp spof http://www.brokenhaze.com/sf_images/router-isp-spof.png
3 - Finally, no SPOF - within your control:
no spof http://www.brokenhaze.com/sf_images/no-spof.png
Now each of these setups will obviously be more expensive than the last. For most small/medium sized companies option one holds the best risk/benefit ratio.
You are going to have to setup dot1q trunking between the switches and the router (BTW did you mean 3825?) and then create a vlan interface on the router. You will not be able to have two router interfaces within the same IP address subnet otherwise.
You may need a switching module in the router for this to function as desired -- such as the NME-16ES-1G.
[edit / additional information]
You will not be able to have two router interfaces in the same subnet unless you either: (a) use a BVI interface as Vatine suggested (there are performance and other considerations using them however) or (b) put the two physical interfaces into a vlan (see example below).
!
interface FastEthernet0/3/0
switchport access vlan 10
switchport mode access
!
interface FastEthernet0/3/1
switchport access vlan 10
switchport mode access
!
interface FastEthernet0/3/2
switchport mode access
shutdown
!
interface FastEthernet0/3/3
switchport mode access
shutdown
!
interface Vlan10
description Server_Vlan
ip address 192.168.10.1 255.255.255.0
!
If you have two routers, then you could provide IP address/gateway redundancy for the servers by using HSRP, VRRP or GLBP.
[edit / additional information (HSRP example)]
interface Vlan10
description Server_Vlan
ip address 192.168.10.2 255.255.255.0
standby ip 192.168.10.1
standby priority 150
standby preempt
!
For your second router, change Vlan10 to ip address 192.168.10.3 and a priority of 140. Use the command "show standby brief" on both routers to confirm HSRP operation.
Best Answer
What's the saying about famous saying about eliminating single points of failure... "Oh, that way madness lies"?
The 4 port NICs on the R710 servers, of which I have several out in the field, are a single Broadcom PCIe device with 4 individual PHYs. A single failure of a PHY probably won't take out the entire device, but a driver going flaky well could. If you're concerned about driver failure you might want to put another, non-Broadcom, NIC in one of the PCIe slots.
I'm running my R710's on VMware ESXi and using 3 of the NICs for connection to the LAN and 1 for the service console. When I get an iSCSI SAN at one of the Customer sites I'll be adding a dual-port PCIe NIC to service the SAN. I've been happy with the configuration, though I don't have NIC-level redundancy for the service console.