8 Total Slots That I understand. Modules must be installed in Matched Pairs Check DDR2 667MHz ECC PC5300 240 Pin Fully-Buffered DIMM mac pro Specific Heatsink on Module Check 72 Bit (64 Bit + 8 bit ECC) I understand that Install 4 DIMMs or More for Best Performance ?? Is this so each bank on each riser has RAM on it? Like if I want to add 4GB to the 1GB I have now, do I need to get 4x1GB, so that I have a total of 6 DIMMS installed? FB DIMM technology takes advantage of multiple channel access. Apple recommends the use of 4 or more modules to take advantage of this 256-Bit Wide memory architecture. Modules must be installed in pairs and each module is 64-Bit Wide + 8 Bits for Error Correction (ECC). If someone could please explain that to me, I'd love them forever. Also, is it okay to add 4x1GB to my machine, keeping the 2x512MB in there, or should I just sell those back to OWC or on eBay and keep the machine at 4GB of totally matched DIMMs? Thanks!