DDR memory single or double sided?

DIMMs are newer and transfer twice as much data as the same size SIMM. That's because each pin on either side of a DIMM is a separate circuit while in a SIMM, these pins form a single circuit.
It also should have labels on the chips.

You aren't so much concerned about whether or not the DIMM is populated on both sides as you are about the "Density" of the memory chips.

The 'best' way to assure that you get compatible memory, is to buy it directly from one of the major memory makers.

www.crucial.com
www.kingston.com
www.pny.com

You can select memory for either name brand PCs, or specific motherboards.

Kingston also has an informative guide to memory that should explain some of the issues.

Chas