OK, works perfectly with the WD Scorpio Blacks as others in the thread suggested. I can't believe the difference compared to the existing WD Blues - which basically are junk - I certainly don't want to put them in my older laptop.
I'll post my experience installing in case it helps anyone else doing the same. I didn't want to reinstall because I've quite a few apps/games installed now. I created my MSI install DVDs just in case, and then created the Windows System Recovery disc. I also didn't want to bother with RAID 0.
Then I copied the 50 GB or so of data I had on D: onto C: and used Windows Disk Manager to resize the C: partition so that it plus the two hidden partitions take less than the space available on one WD black (about 460GB). I used some freeware partition copying software to copy the partitions (excluding the empty D: ) to the new disk.
Removed the existing disks and put the new disk in one of the interior bays, restarted and... zip, nothing happens. So I used the Windows System Recovery DVD and opted to repair a Windows installation on the disk (even though it couldn't identify it). This fixes the MBR on the disk. Restarted and got the Windows boot loader ("missing hardware" error) and then restarted again with the Sys recovery and this time it fixed the bootloader references. Final restart, everything perfect, just as before (except even Windows seems to be faster to load).
With the second disk installed, I even had Intel rapid storage ask if I wanted to set up RAID (it seemed to suggest I could expand onto the second drive without losing my existing install on the first disk). Nevertheless I'm just using the second disk as D: (data, games).
Anyway, quicker IMO than a fresh install.