Figuring out what router is necessary for a given bandwidth requirement is hard because Cisco doesn't publish the maximum bandwidth potential of their routers. The only performance metric they tend to give is packets-per-second forwarding capability with 64-byte packets. This does little to help answer the question "how much bandwidth can a given router route" because the average packet size in any real network is (almost) always going to be significantly above 64 bytes, and the packets-per-second capability of any given router tends to drop substantially as packet size increases.
So as end users we're essentially left to performing our own bandwidth tests on our routers and sharing the results. We can then make some approximate guesses at the peformance of other routers based on their pps ratings relative to the pps ratings of routers that we have tested. But these are only estimations and when it comes down to it you can't know for sure what bandwidth a given router is capable of acheiving until you test it.
Two quick examples (and if you give some more details about your bandwidth requirements we may be able to help further):
The Cisco 2621 is rated at 25k pps, which comes to about 12.5Mbps with 64 byte packets. Per my own testing, the 2621 is capable of around 40Mbps with full-sized (1500 byte) Ethernet packets. The 3620, rated at 20-40k pps (10-20Mbps) can do around 75Mbps or so per testing done by someone other than me.
So it gets to the point where you can look at a given router's pps rating and maybe hopefully be able to kinda ballpark the bandwidth potential. But it's far from an exact science, and the "bandwidth capability with full-sized packets" numbers aren't the most useful thing in the world anyway since very few routers have only full-sized packets going through them. And it gets even more complicated when throughput-reducing features like ACLs need to be taken into account. (The tests above were done with no such features enabled. CEF was enabled.)
So, there are all these things to consider and for the most part it's all just guesswork. But it's about the best you can do unless you're fortunate enough to be able to get a loaner or two beforehand, configure them as they would be configured on your network, and test their bandwidth capability with traffic streams that roughly match the traffic that is currently flowing (and/or what you're expecting to be flowing in the future) through your network.
