Hello and welcome. It's been a while since I have had to do much to my 630i so here is my 2 cents worth.
As far as CPU's go, you can get a Core 2 Duo quad core, not sure which one, my 630i has a E8500 in it that was sufficient for what I was doing. If you can find one, I don't think it is worth the amount of money you would have to spend on a processor to upgrade. The E8400 is pretty good for what it was.
I did use a Nvidia GTX 660 Ti in it and it worked fine.
You can't use DDR3 memory.
Yes, flash the BIOS, I think version 1.0.13 is the latest and I believe it is here on the website located resources/driversandfirmware.html
also flash the MIO firmware if you never did that.
My only other piece of advice to you is to upgrade to windows 7 if you can get a copy. At least it's still maintained by Microsoft and it worked pretty good on the 630i.
Hope this info helped some.