Sorry, but the P5VD1-X does not support any of the Core 2 Duo processors at all. (This includes the Pentium Dual-Core processors, as opposed to the Pentuim D processors.) This is due to hardware compatibility issues between the motherboard's VIA PT880 Ultra chipset and the Core architecture. As a result, if you try to put in the Core 2 Duo on that board, your system will likely refuse to even POST - and you will be stuck on a blank display indefinitely, forcing you to yank the power cord just to even turn the computer off.
Thanks for your reply. It's a shame. But I guess I can always buy a Pentium D, right?
You could - but Intel has not manufactured any such processors for the past two years. (Intel had already EOL'd all of its Netburst processors, including the Pentium D, in mid-2008.) As a result, you'll have to buy that processor used or NOS (New Old Stock).
And yes, the P5VD1-X processor compatibility as I stated in my previous post comes directly from the specific motherboard's CPU support list on the Asus Web site. It has no Core 2 processors or Pentium Dual-Core E#### series processors on that list at all. And this is largely because Intel and VIA are in a licensing battle for its core-logic chipsets, resulting in VIA not updating any of its chipsets to work with the Core architecture CPUs (and VIA has not introduced any new chipsets for Intel platforms since the days of the Pentium 4).
By the way, if you want to see really improved performance, I'd strongly recommend that you consider replacing/upgrading the motherboard, processor, memory and possibly the graphics card. This is because the Pentium D does not offer sufficiently improved performance over the regular Pentium 4 in most programs to justify its current going price.