To add a few more points....
SSD vs. platters:
I will stay with platters because of the price, also SSD's do not have the big range of the platters.
Ram:
Get as much as you can afford and the amount the motherboard can take it will make your system run smoother and in so doing you will have less blue screens of death.
Power supply: Depends on how much you have inside your system, hdds's video cards etc.
CPU: This is debatable and also dependent on the motherboard.
Backups: This is a must have. Always have a backup in place, on-line or another hdd (on-board/external). I go for a small backup server with a Raid 10 but this is not everyones taste.
Malware protection: This will depend on what operating system(s) you have, but for a Windows system it is a must have.
