We are a small, new company and do not have an IT person yet so I need some basic recommondations. We get an OpteMAN connection from AT&T which comes with a CISCO 1841 or 2821 router (depending on speed). What are the advantages of the 2821 over the 1841 and visa versa?
We mainly need a fast connection for multimedia data but will also use it for IP phones. In addition we need VPN access. For the later should we use the Cisco router or could we use a separate (cheaper) VPN router (behind the Cisco router?)
In general the differences between the 1841 and the 2821 are differences in processor speed (2821 faster), in memory (more on 2821), the 2821 is rack mountable while the 1841 is not. The 2821 has more slots for network modules. I believe that the 2821 has an option to do power over Ethernet. If you are going to use IP phones this could be an important factor favoring the 2821. If you were purchasing it the other difference is price but if it comes as part of the package from the provider the price of the router is not of concern to you.
As Rick points out, the 2821 generally either has more or can have more over the 1841. So, I would normally lean toward the more capable box, all else being equal.
You also mention possibly using IP Phones. If I'm reading the specs correctly, voice isn't directly supported by the 1841 but can be by the 2821 (might require additional hardware and/or additional software licensing).
With regard to your second question, either Cisco router, I believe, will support VPN. However you didn't mention what "type" of VPN you plan to do, so hard to say whether it will support what your looking to do (likely it could). One thing to keep in mind might be additional licensing costs to use the VPN capability. Cisco routers provide different feature packages. Perhaps this too might be covered as part of your "package", but you might ask what software will be on the 1841 or 2821.
If the router is being supplied by AT&T, don't forget to ask about who maintains it and options for supporting voice, VPN etc.