Never mix make and models. If you want to keep your sanity, always install all the RAM in a PC from the same make and model. If you can, keep it to the same manufacturing batch.
There's a 24Go Ram package (6x4Go modules) currently available on eBay.
But 5 of them are from Crucial, and the 6th one from Transintl.com.
Does anyone know if these 6 modules can be used together, despite being from two different manufacturers?
Full tech specs here: