P.O.S.T. Freezes at Memory Check on 1st Boot.

Here's my situation; I have a Velocity Micro pre-built system w/Asus P5N-E SLI 650i Mobo. With my existing 4 1G sticks of Corsair PC2-5300 (667mhz) Ram, all is good. When I remove them and install 2 2G sticks of Corsair PC2-6400 (800mhz) RAM, my system freezes at the memory portion of the POST on the first boot of the day... or after the machine has been down awhile. Re-boots (and boot-ups after only a short down time) work fine. The freeze only seems to occur after the machine has been off for awhile. There is no O/Cing; all is stock. The memory showed no errors after 7 passes with Memtest86. The CMOS battery is new.

I have tried leaving the BIOS settings at "Auto", and have tried "Manual" as well. I have the memory voltage set to the recommended 1.8v and the timings set to the recommended 5-5-5-18 2T. This is the 2nd brand of 800MHZ RAM I have tried, and I have had this same problem with both. The BIOS is a late version designed for 800mhz memory and 1333 mhz FSB, and it recognizes PC2-6400.

Why the old ram? The 667mhz ram works fine. It's the new 800mhz stuff I'm having problems with. Nevertheless, I tried increasing the DDR2 800's memory voltage to 1.9xx volts... but Vista froze after about 10 minutes. That never happened before!

I just set the timing specs all to 'auto' to see what that does. (Unfortunately, now CPU-Z shows that the tRAS is 31 clocks.)

My post should have read:
Put the old RAM back in, increase voltage, then put the new DDR2 800 back in and see.

Anyways, like zip said, you really are not getting any performance boost unless you plan to use 2x of the old 1GB RAM along with the new DDR2 800.