Depends what you put with it?
I would personally go 350 or even 400watts (the extra processor will use an additional 70watts)
Also I have some vague recollection to the use of an additional power cable on the Tyan Tiger motherboard,
I may be talking rubbish but buy the Moby first and check.
You also have to be careful with the size of case you buy for these boards (they tend to be very large) and not too many standard cases will accomodate them, also be careful as to how long the power cables are because if you end up with a full size tower this combined with the sometimes low position of the ATX power plug on the board can mean some aren't long enough.
Just remember a friend having a few of the above issues on a recent server build with a Lian Li full tower and Tyan Tiger. Martin Just trying to help, sometimes falling short, I am only human after all.