"DOS" is a strange term in this context. Although Windows 98 was still based on DOS, it's totally gone in Windows XP and later. Do you happen to mean the BIOS.
Are you sure the new CPU is on the motherboard compatibility list? And supported by the version of the BIOS that it has? What exactly is a '2 quad core 2006'? Your post here is the only reference Google finds to it. What was the old CPU?
It only boots from the new hard drive if it has the right OS. You didn't tell anything about that. If there is no OS on it, it will try the CD, which explains one of the errors. So better clone the old hard drive to the new one and try again.
If that doesn't help, it's best to go totally back to the old situation with all of the old parts and none of the new parts. Get that working. Then replace the components one by one and test after each replacement. Start with the CPU. Then the power supply. Then the hard drive with the cloned content. Then add 1 graphic card and get it working by installing the drivers. Then the second graphic card.
It seems to me it should have been easier to buy a new computer in stead of doing such a big update of one that's probably 6 years old or more. But that's your choice.
I've been trying to fix this for 3 days now, am self taught and need to fix my pc so I can work again.
Issue: I replaced parts in my Dell XPS 630i desktop. Now it will boot to DOS but then shows errors if you try to boot further then that.
Errors:
-Floppy Disk(s) Fail (40)
-CMOS checksum error - Defaults loaded
-Audio Cable Not Connecting
-USB1 Cable Not Connected
-1394 Cable Not Connected
-Front I/0 Cable Not Connected
-Boot from CD: No boot device available
Replaced Parts:
-Power supply is now 'LEPA N500'
-Graphics card1 is now 'ATI Radeon'
-Graphics card2 is now 'Gigabyte D33006'
-Hard Drive is now '2TB Western Digital'
-CPU is now '2 Quad Core 2006'
All cords are plugged in, as are the parts too. Its set via DOS to boot to the hard drive which it shows is there, not to the CD drive. I don't know why it is gving an error for CD booting, or what these errors mean. I can answer all questions.

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