When I was recently researching MB's for an upgrade I was looking at getting a new board that would support PCI-X. What I read was that the PCI-X boards do use the new 24 pin power supply connectors, but you can still use the old 20 pin connector with an adapter ($5). Some people said you don't even need the 20 to 24 pin adapter, but at such a low price I think I'd get one just to be on the safe side.
I think part of the question is whether or not you will want to run two vid cards in SLI. For me, the ability to upgrade to SLI in the future was the only reason for considering a PCI-X board over an AGP board. Not all PCI-X MB's will do SLI but the cost for an SLI board is not much over the non-SLI boards. The reason I mention this is because if you are looking to run SLI, then a 430 watt PSU may not be enough power. SO, you would have to get a new PSU anyway to run the two vid cards and at that time get the "SLI ready" PSU.
Finally, there was one post I read where the poster stated he had spoke to an MSI rep who told him that while you can run the new boards with the 20 pin connector - it isn't really "good" for your system in the long term. Knowing most CSR training, chances are good you could get 5 different answers from 5 different reps...
