Disaster Recovery: How can I configure multiple default gateways so that one sever outage doesn't mean loss of net access?

We have a network with a leased line coming into the office to a router that is managed by our ISP. We then have a sonicwall attached to that router with an IP of 10.0.20.1. The SonicWall has two machines attached to it, an Exchange Server and a DC / File / DHCP / RAS etc server. They are both dual homed, the external adapters are attached to the firewall but the internal adapters to our 24 port switch.

Sonic: 10.0.20.1

DC / Gateway
External: 10.0.20.5
Internal: 10.0.10.1

Exchange
Int: 10.0.10.2
Ext: 10.0.20.6

Right now the clients are using 10.0.10.1 (DC) as a gateway. It is also a VPN / NAT / RAS server. I am trying to add resiliancy to the network by making it such that the loss of one server doesn't take out our net access. This is especially important because our servers are offsite (connected via sonic to sonic vpn) and we need access to our customer db, etc.

I have set up secondary DNS on the exchange machine and it syncronises with the DC so that if DNS crashes we're still in good shape. I've set it up so that the DC handles DHCP for 10.0.10.1-200 and the Exchange Machine can handle 10.0.10.201-254 in case DHCP gets cocked up. But what I really need to do is configure the Exchange as a second gateway and pass this to clients via DHCP, because now whenever the DC needs to go down for one reason or another we lose net access and therefore access to all our collocated machines. How do I do this? Should I install RAS on the Exchange server and configure it like the first (NAT, etc)?

I would also like to mirror the files we have on the DC (we have team file storage) on the Exchange in case of a critical failure--is there a way to make data sync between the two that doesn't require add on components (or cheap ones). We're going to add a secondary DC as soon as we get the cash, but for now what's the best way to prevent downtime if one of the machines goes down?

Jay