Which router should I get and how should it be setup?

We currently host 20 servers (Vmware clients) in-house with a cisco 800 router, servers are configured with 2 network cards, LAN and WAN IP address. Firewall is configured on both router and windows OS.

We want to move the setup to a co-location datacentre. I am thinking of using a NAT so that there wouldn’t be any need for WAN addresses on the servers. The number of server is being reduced to 4. So I thought I could have one Cisco 870 setup with VPN, Firewall and DMZ. And maybe another Cisco 870 as a cold standby.

Was wondering if anybody thought this was ok or if there was better solution, without paying an arm an a leg for a PIX.

Thanks