You'll find the CPU maxes out (on an 1841) when pushing through the full 40Mbps and using statefull inspection and NAT, though it can sustain the rate and I'm using it day to day quite happily.
1801 is only rated a few Mbps lower so should be similar, though I don't think you'll get away with an 80/20 connection. Any QoS might also tip it over the edge.
3620 isn't going to cut it, it's got half the rated performance of an 1801 at best, a quarter at worst.
I've seen a 2821 manage a lot better, though unlikely to be a router of choice for what I suspect is a home environment?