It seems strange that your firewire external would have been unbootable. I bought a Maxtor OneTouch 4 Mini 320gig (owned by Seagate) and I cloned my drive to it, and it boots through usb. Its a little slower, but it boots, and runs normally, so I wonder if maybe you had a defective cable.
Speaking of WD, I did get a ScorpioBlue (5400) 500gig drive for my MBP, and it's a stellar drive. So far, WD seems like they're on their game.
I thought of doing that, but it would take forever on a 500gig drive. What's the benefit of zeroing on a new drive? Wouldn't cloning a couple hundred gigs to the new drive give it a good workout - uncover potential problems also?