Checksum error can show up for hard drive issues as well.
I am going to throw a few things your way just in case one of them brings you to enlightenment
You motherboard needs the main power, the 4 pin power near the CPU, and in SLI the EZ_Plug 4-pin Molex power connectors all connected.
In SLI it also needs the ASUS EZ Selector Card installed for SLI. This is that funny card that sits between your x16 Slots.
The Checksum and non-booting could be your RAID0 array, did you unplug the SATA Connectors? If so it's possible you plugged them back into diffferent SATA Ports which messes with the way the controller handels the drives, gota keep them plugged into the same ports once the Array is built.
You should have sufficient power from that PSU, you could put everything back pre-upgrade and see if everything checks out, then step through your upgrade a peice at a time, ie.
I saw some mentioning in your motherboard manual about some of the defaults. So once you cleared your BIOS and reset to defaults you might have some odd settings that are conflicting. Something you had to set before and forgot about this time.
And last, some motherboards hate it when you have the BIOS set to look for a floppy that isn't installed.
Best at the moment.