The Abit board will work just fine if it has the current bios, it has external esata that I like and cool shiny copper heat sinks for cooling.
The DFI is less costly, and if it too has the current bios it will work just fine.
I would not bother with an X series board, really no need for one over the P series.
I would go with the DFI myself, but I use an after market HSF Arctic cooling freezer 7 pro. which will not provide the needed air flow over the nice shiny copper heat sinks of the Abit board. It's made for the reference Intel HSF.
The big problem is making sure you are getting the needed bios with the MB to support the CPU you wish to use, this is where buying from a local shop comes in handy.
I on the other hand keep a spare cheap skt 775 celeron for use as a boot into bios cpu. I have used it 2 times so for me it's saved time and money.
I would be interested to see how, with what ever configuration you come up with, the Q9450 does.
I would probably just go with the DFI and a Q6600 if I were going to use memory like that which you have on hand, save a few dollars and still get a good system out of it.
Good luck and always use a high quality power supply.