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Software router performance, such as for an 871, very much depends on what your traffic looks like and your device configuration. Cisco notes the performance for the 860 (same 25 Kpps as the 870 series) ranges from 16.8 Mbps (min packet size) to 198 Mbps (max packet size) - that's just packet forwarding. (Cisco also notes the 860 series can deliver up to 27 Mbps for IMIX traffic, @75% CPU, and with PAT.) Cisco recommends an 860 for up to 4 Mbps (duplex) bandwidth.
The 890 series is recommended, by Cisco, for only 15 Mbps (duplex), but it's also noted as delivering 45 Mbps for IMIX traffic, @75% CPU and with NAT/QoS/ACL. I.e., the 890 might work for you.
BTW, for 45/5 (or 25 Mbps duplex), Cisco recommends a 1941 or 2901.
