Peter,
SiS' web site doesn't say that the 648 is compatible with 2X.
nVidia's web site doesn't say that nForce (1 or 2) is compatible with 2X.
Likewise, as you mentioned, Intel's site doesn't say that its P4 chipsets are compatible with 2X.
This leads me to believe that these chipsets do not support 3.3V cards. Especially since I'm not aware of any boards based on these chipsets which support 3.3V.
In any case, I'm referring specifically to KT333 and KT400. According to VIA's site, these chipsets are compatible with 2X. However, chipset compatibility doesn't guarantee that a particular motherboard will work with a 3.3V card. It depends on the type of slot used on the motherboard, as you alluded to.
My question was: which KT333 or KT400 motherboards (not chipsets) will support a 3.3V card?
I know that AOpen's KT333 and KT400 boards are keyed for 1.5V. This is also the case with Asus' A7V8X and (I think) their A7V333.
But what about other boards?
I was hoping that owners of KT333 or KT400 boards would sound off with regards to their particular boards.