I used to have a very handy poster which had all of the Cisco equipment in matrix form which laid out which router had which interfaces and slots, with the same for the switch lines.
Is there a web-based equivalent of this? It would be very handy when trying to decide which networking equipment I need to buy.
Here is their small business product line
Here are some enterprise offerings
http://www.cisco.com/en/US/prod/collateral/routers/ps380/prod_brochure0900aecd8070826d.pdf
Not answering the need of physical details like interfaces and slots, but still useful, this pdf gives a quick look at cisco's numbers in terms of routing perfomance for their different routers, from old 1400s to 12k and CRS-1.
Quick overview:
World Data Products offers a free poster similar to what you're asking for if you're willing to give them your contact information (which they will use as a sales lead, but their prices have been pretty competitive). Unsure if it's been kept up to date, but I had one a few years ago and it came in handy.
