Using Danilo's reference, you'll generally want to use the Fast/CEF Switching Mbps rating. Remember, both to leave headroom, perhaps as much as 50%, and remember to account for both directions. For instance, is the 20 Mbps you note 10 Mbps duplex, as if connected with 10 Mbps links, or 20 Mbps duplex throughput, as possible on 100 Mbps links? If the latter, to play safe, you'll want a router that has a rating for 80 Mbps or better, e.g. the 2821, but if the former, a 2801 should suffice.
Also keep in mind, whether you'll need to sustain a certain throughput, or you might burst to it. If the latter, you can use a less powerful router, e.g. even the 2801 might be sufficient for bursts to 20 Mbps, even duplex.
PS:
Since you've only mentioned Ethernet and LAN bandwidth throughputs, if this router is just for LAN routing, you might also want to consider a L3 switch. You'll need to carefully insure features are acceptable, but something like the Catalyst 3560-8PC offers much more raw performance than any 2800 series router. Its price might be attractive to you too.
