Hi Artur,
Please don't get offended with this response.
When you are reading Data Sheets from the Cisco website, always take them with a ton of salt. The data sheets were written by MARKETING folks whose sole intention is to sell. Every manufacturer will BLOAT the performance of their product.
The figures quoted in the data sheet are expressed in half duplex (one-way traffic) and without any encryption. The "bible" a lot of us are using to "size up" which routers are suitable to the appropriate WAN speed is this: Portable Product Sheets – Routing Performance.
Take note of the following items:
1. Like the data sheets, this PDF file was updated by the MARKETING team;
2. This PDF file is no longer being updated. The team that was tasked to do so got "dissolved" back in 2011 during the annual "re-organization".
You say that your have an 80/20 Mbps DSL link. Let's say, average of 50 Mbps.
Go through the data sheet and look for a router which can, more or less, push up to 200 Mbps (right-hand column) and you'll reach the 2951.
Now be aware that, for example, in the next two years you plan to upgrade your WAN link to a full-blown 100 Mbps ethernet link and you are hoping that your trusty 2951 can support this. The answer is no. This is why you need to look at your current requirements and your future requirements. If you plan to go up to 100 Mbps, again as an example, in the next two years then look for a router that can push between 350- to 400 Mbps.
So if you have a 100 Mbps link, why should you look for 400 Mbps? Because you count 100 Mbps as one-way traffic. 200 Mbps means you have two one-way traffic of 100 Mbps (aka full duplex). Next you can't encrypt one-way and do not encrypt the other way. When you run encryption, you run encryption BOTH WAYS. So you take 200 Mbps and double this. Hence, you get 400 Mbps.
