When I built my PC the memory I chose was 2.1V but the motherboard is 1.8V. I increased the voltage but have occasional memory errors so I have purchased a 1.8V set. My question is should I decrease the voltage before installing the new memory or just wait for the first boot and then enter the BIOS.

This totally depends on the memory type. You haven't mentioned the memory make, speed or type at all, have you. Most motherboards will detect the memory type and set the speed/voltage automatically

Sorry, I did not think this was going to be a complicated question since by manually changing the BIOS voltage to 2.1 I believed I removed the ability for it to auto configure. Here is the whole story.

Motherboard, Gigabyte GA-EP45C-UD3R
Intel Core 2 Duo E8500 Wolfdale 3.16GHz
Memory, OZC Reaper OCZ2RPR11502GK
Video Card, Gigabyte GV-R485OC-1GH

After building the system at the beginning of the year a system error report advised me to download and run windows memory diagnostic. The diagnostic was detecting a large number of failures. By using my two DDR2 modules one at a time I had isolated the problem to any module installed in socket DDR2-3. All other DDR2 sockets test fine with both modules. The memory was purchased based on it’s listing on the MOB QVL document.

After contacting MOB support the BIOS settings were changed for the DRAM voltage to 2.1V per the memory spec instead of the 1.8V that the MOB automatically used. I was still getting memory errors on DDR2-3 so I opted to replace the MOB before I ran out of RMA time. New MOB did the same thing but by using the DDR-2 & 4 sockets there were no more memory errors with MEMTEST.

Over the past couple of weeks I started to get the error message in the attached screenshot. So I have decided to replace the memory modules with Crucial CT2KIT12864AA1067 based on the selection tools at the Crucial web site. They have already arrived thanks to Newegg’s excellent shipping so I will go ahead and lower the DRAM voltage or reset it to auto prior to installation since I do not want to blow the new modules up.

Yes the 1.8 volt memory voltage should only be changed a step at a time. My OC'd Intel Core 2 quad is running from stock 2.4GHz to 3.51GHz. The 4GB 667 memory is running at 870MHz with the memory voltage set at 1.83 volts. So you see 2.1 volts is rediculous. Install the new memory and leave the motherboard at 1.8 volts, stepping up one notch at a time if you need to

I would probably change the voltage before installing the new RAM. Call me superstitious. The BIOS may have an "auto" setting anyway. If it's there, just reset to that. If it's not then drop the voltage with the old RAM installed, power down and swap it.

I reset it to auto and ran two passes of extended MEMTEST last night with no errors. I will wait and see if the above application error happens again.

Well still getting the above error. Anyone want to recommend how I identify the location of the problem?