Although Cisco recommends 3845s for full T3s, the 3825 generally works well for them too. For a full T3, even a 2851 might suffice.
For a single or few bonded T1s, the less powerful 2800 ISRs, 2801 or 2811, generally serve well.
The above addresses just performance, but you might also need to consider how else you might want to expand the capabilities of a router when selecting one, expecially with regard to interfaces or other modules.
http://www.cisco.com/en/US/products/ps5854/prod_models_comparison.html
http://www.cisco.com/en/US/products/ps5855/prod_models_comparison.html
If you don't see using a feature within the next two years, consider not buying it now, but buying in when needed (likely newer/cheaper routers at that time).
Within the 2800 series, don't worry about gig Ethernet, none of the 2800 routers can handle substained gig.
