Lol, I "solved" the boot issue. The customer already had one Seagate ES 500GB disk that he wanted to attach to his new comp.
So I added the drive and just for the heck of it set the Seagate to #1 in the HDD boot list. Guess what.. the VR Raid 0 with Vista on it, now in 2nd position, boots up fine, without any delays or errors (since the Seagate doesn't contain any OS).
If I change the priority back to the Raid array being #1 I get "boot disk error" again, perfectly replacable. I cleared CMOS several times and even reflashed 1002, so no OC problem and no corrupt bios

Neither ideal nor logical solution, but it works. Better than having to leave Vista in ur drive.

By the way, I'm running 1700/2000 unlinked with a Q9550 at 3,6Ghz and 2x2GB CSX Diablo, P1 & P2 enabled (needs 1,54V Vnb bios and good cooling though, water through Fuzion block + MX-2 under the hood in my case).

Sadly I had to RMA the last Diablo D9GTS kit I got, as it couldn't do more than 1600 C7 without errors. CSX rep said their last batch of GTS evidently contained some bad samples and they were out of stock on Micron-based Diablos, so I had to use some Samsung for quick replacement.
They run 2000 at 1,9V for sure, CL10-9-9-24 kinda sucks tho