DDR2 wasn't really around long enough for there to have been low and high density variants, 2004-2009 for Intel.
As opposed to DDR3 which was around so long that there were 3 common densities--2Gbit for Core 2 including G31/G41, 4Gbit up through Ivy Bridge, and 8Gbit for Haswell and all AMD. So the seller was probably referring to DDR3. What this means is on G31, you cannot install any higher than 4GB sticks whether DDR2 or DDR3, and those must have 16 chips on them (generally 8 on each side hence "double-sided").
Which is academic because G31 cannot remap addresses above 4GB anyway, so cannot actually use any more than ~3.25GB of RAM (the rest will show up but as "Hardware Reserved") whether or not you use the IGP and yes, even in a 64-bit OS. So just about any non-registered (unbuffered) DDR2 2GB sticks you can find should work, just don't expect to be able to use all of it. And keep in mind that Intel desktop chipsets of the era could not run lower multipliers for the memory than 2.0, so a 1333FSB processor requires at minimum PC2-5300 DDR2-667 speed RAM to work.