What BBRV/Neko say in the powerdeveloper.org thread, is that Genesi shipped many Pegasos2 systems with 2x 1GB memory modules, but
1) You may need a 2B5 revision board for it to work (the last revision)
2) You may need paired RAM (identical memory sticks, paired by the manufacturer)
3) You may need memory of a certain type. I guess the only way is to try? Genesi used "Kingston kit ram with blue heatspreaders", model number KHX3200AK2/2G