I once did a chkdsk /f on one of my laptops which reported a number of bad sectors, but after restoring with a system image for which I can't remember now, when I ran another chkdsk /f, that came back clean.
As hardware as always trumped software, I was at loss as to how that happened.
I took the word of HDS over the other methods I'd used because the number of bad sectors were spiralling which each HDS test.
That HDD was a 640GB one with quite a lot of free space on it, but because I wasn't experiencing any symptoms of a failing HDD, I didn't know where HDS was finding these bad sectors.
Anyway, I decided to err on the side of caution and replace it.
You can pre-empt a failure by ensuring you have a system image and a system repair disk to boot up with should you need to image a replacement HDD.
You haven't said how you knew you had a bad RAM module.