5 and 20 Mbps bandwidth? What kind of WAN bandwidth is this? If Ethernet, you might also consider small L3 switches, either the LAN variants, 3560 and 3750, or Metro Ethernet switches, e.g. 3750 Metro Series. These will easily handle much, much more bandwidth (multi gig), but you do need to insure the features they provide, which are often much more limited than a router's, suit your purposes.
The ISRs you've selected for hub and spokes at 20 (3825) and 5 (2800 - 2821?) Mbps, certainly fall within Cisco recommendations, but many of Cisco's latest routers can support more bandwidth than Cisco's recommendations. For instance, Paolo's recommendation of an 1841 should be good for your 5 Mbps spokes, and it's a modular router (an important consideration - and likely a reason he recommended this model). However, using Collin's reference, for 20 Mbps, you might want something with more performance than the 1841's 75 Kpps, especially since we often try to avoid running a software router routinely at or near 100% CPU load. You've already mentioned the 3825, Cisco recommended for half T3, making it the "safe" choice, but a 2851, or even 2821(?), might work well for you too. You didn't note what you might expect to upgrade your hub site to if the spokes are 20 Mbps, but up to about 100 Mbps, Paolo's suggested 3845, although more than the Cisco's recommendation for T3 bandwidth, I think, is likely to be fine.
Something to consider besides raw performance, is what might be added to the router. Besides CPU performance, the various 2800 models support different numbers of DSP slots, modules and module types. You might want to look over the various network modules, and if there are any you might want to use, you might consider choosing a 2800 with the number of and type of NME slots you might need.
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Cisco peformance recommendations:
"Q. What is the performance of the Cisco 1841?
A. The Cisco 1841 is a new-generation, best-in-class router platform designed to deliver multiple concurrent services at wire-speed performance up to single T1/E1/xDSL speeds."
2800 series, see Table 1 in: http://www.cisco.com/en/US/prod/collateral/routers/ps5854/prod_qas0900aecd80169bd6.html
3800 series, see: http://www.cisco.com/en/US/products/ps5855/prod_models_comparison.html
