Bit the bullet and bought a 150G Raptor - more thn enough for FSX, even with Gex :) just kidding.
I backed up the FSX array with Ghost so I'm hoping I can restore it to the 150, which is more than large enough. Would hate to have to completely reinstall FSX/SP2/Accel unless absolutely necessary.
I'm thinking that booting from the CD and doing a repair install of xp might get me thru the mobo change without reinstalling everything but we'll have to see.