RAM running at a lower frequency

Most likely it's 666Mhz (1333MT/s). Some processors are not capable of 800Mhz (1600MT/s). The 2nd gen i3 processors in the Vostro 2520 (and most other 2nd Gen core i CPUs) do not support 1600MT/s, only 1333MT/s.

An example, the i3 2328M from a Vostro 2520:

http://ark.intel.com/products/70927/Intel-Core-i3-2328M-Processor-3M-Cache-2_20-GHz

Memory Types DDR3 1066/1333

Some 3rd generation and later CPUs support 1600MT/s though.

Also note the "multiply by two" has nothing to do with "dual channel mode". The multiplying by two occurs because it's DDR RAM. DDR stands for double data rate.

Its highest common frequency. If you don't pair the DIMMs they'll operate at the best they can both do. Difference between 1330 and 1600 will be of very little impact unless you're doing really memory intensive tasks.