i am using a 1tb drive and it shows just shy of the 1 tb mark which is common it is a Wd drive. I don't have it hooked directly to the ps3 I could never figure out how to have the ps3 see the drive so I leave it on the network and just ftp the games back and forth.
You can safely purchase that 1TB. I have a 1.5TB working just fine now for 8 months. I think their problem was when they formatted the hdd. My first attempt to format the 1.5TB resulted in the same problem. That was resolved using a good format utility - Can't remember though if it was Swissknife or Partition Logic. But there is nothing to be worried about!
Swissknife is the way to go - I'm using multiply external large drives with my PS3s and apart of a few USB sticks not being recognized properly by PS3 regards it's size / avilable space I had no problems using those once partitioned and formated properly.