I didn't think you could mix the Ram, but thought I'd ask anyway.

I did believe that you could run 4 HDDs (cool).

My thought is: for the price of the mobo and cpu, I could make a significant processing upgrade, while continuing to use existing HDDs, Video card (AGP), and Memory (DDR).
Later, I could swap out the memory completely, to DDR2.
Then, replace the Video card.
And add Sata drives (making the Sata the OS & Apps and using the IDEs as backups)

And it sounds overclockable, bonus!