I have a C521. It came with a single core 2.0 processor and 512 of ram. Replaced the 80 Gb HHD with a 500 Gb HHD. Did a wipe drive to the 1% level and quit.Replaced with 4 512 Gb ram. But I replaced each stick of ram singly. Turned off computer ,then added the next 512 of ram and so forth. I did this before I loaded Win 7 32 bit Ultimate OS. Loaded the Win 7 and all the drivers were found. Then added a 8300Gs video card and updated the driver. Works like a charm. Has windows index of 3.9. I am going to replace the processor with a AMD AM2 Athlon 2.3 dual core, because processor has an index of 3.9. Computer will run like a champ. Had everything just lying around except for the processor which cost me $8.00. Computer will function just fine. Works great right right now. Connected to TV.