I recently I found a deal for Transcend memory (from the forums) for 65$ for 2Gb chips, so i purchased two from newegg for my 2.66 Quad. (this here: http://www.newegg.com/Product/Product.aspx?Item=N82E16820208200) I have 4x512 Stock memory plus an additional 2x512 from a different 2.66 (they upgraded all theyre memory and i got the 2 chips from them) i get the 2x2GB chips today i put them in, the bottom last 2 slots and boot up into all the OS's run some apps and all seems to be working fine. Then i start running Rember and then after about an hour or so i get a kernal panic and then my system starts taking forever to boot, then kernal panics on start up, first time ever. i remove all the memory and test the new ones with Apple HArdware test (extended test) and they turn out fine, no issues reported, i boot into the OS's (10.4.11, 10.5.4 and XP Sp2) and works like a champ. I test the Stock 4x512 chips and they test fine I test the 2x512 obtained chips and they test fine. is there a real issue with having 6x512 + 2x2GB in the MacPro at the same time? and is there a recommended configuration with them with this amount of memory? (my set up is this with the 7GB) Riser A Slot 1 - 512MB (Stock Riser A Slot 2 - 512MB (Stock) Riser A Slot 3 - 512MB (Second Hand) Riser A Slot 4 - 512MB (Second Hand) Riser B Slot 1 - 512MB (Stock) Riser B Slot 2 - 512MB (Stock Riser B Slot 3 - 2GB (Transcend - new memory) Riser B Slot 4 - 2GB (transcend - new memory) thanks in advanced for any suggestions!