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Good news, bad news . . .
Actually real production capacity of many Cisco devices is often higher than the forwarding bandwidth capacity specified since that is often quoted for minimum sized packets. I.e. With "normal" traffic you're likely to be able to push more than 38.40 Mbps on an 1841.
However, forwarding bandwidth capacity is total, and WAN links (and most current Ethernet) is duplex. For example, your 100 Mbps Metro Ethernet could want to push 200 Mbps through the device. Further, the forwarding capacity bandwidth doesn't account for additional "services", and such can dramatically impact actual capacity. I've found if you divide the forwarding capacity by 4, you're usually okay. So for your 38.40 Mbps, anything up to about 9.6 Mbps should be okay. Your 1841 is unlikely to have had an issue with your 10 Mbps MetroE but I wouldn't recommend it for 100 Mbps.
I've attached a Cisco document that discusses performance of the 2900 and 3900 series ISRs.
PS:
BTW, if your not using any services that are found on routers vs. switches, you might find a small L3 switch, whether LAN variety or MetroE variety, suitable too.
