The P965 northbridge wants its memory close the Intel-specified voltages of 1.8V. It usually doesn't have problems with 1.9V, either. But when you get up to 2.0V memory, it doesn't want to boot it because its too high. So, you have to either RMA the perfectly good RAM for a lower-voltage one (either slower, or higher priced generally), or you have to buy/find/borrow a 1.8/1.9V DDR2 ram stick to get into your bios and set the vdimm to the voltage specified for your ram. This is a reason that OCZ ram appears to have compatibility issues..all of its ram takes more voltage it seems, so the boards lock out until you set the vdimm higher.
You could take your computer to a local shop and pay them to help you real quick, go to your local high school or college (They HAVE to have a stick of slow, OEM ram from a Dell or HP computer they have), or else you can buy a cheap 533/667Mhz DDR2 256MB module to use for diagnostic purposes, most of those modules are the specified 1.8V.
EDIT: Yes, they ARE expensive. Ugh. Gskill GBHZ ram used to be like 30 bucks less per GB. Then their sales went through the roof and they became "premium" sticks. This should slow down as DDR manufacturing switches over to the new standard as well as the "glut" of DDR2 switching takes place. I hope.