4 x 2GB ordered and delivered within 3 days to get. Were the best 2 GB bars DDR2 memory that I could find. The Dell PCs (Optiplex GX260) that have been so upgraded to 4GB RAM, its memory can address only up to 667 MHz according to the manual anyway. That these are displayed in the BIOS and by SiSoft Sandra only 533MHz, I reckon to the PC. Also, I think that you can not perceive the difference, and for this price (about 17, - per 2GB module) would be me 533MHz RAM right. PCs running error-free for a month (storage technology).