DDR2 800 Kits not Cooperating

So, I'm using 2 dual channel 2GB kits (4 DIMMS) of DDR2 800, both rated at 4-4-4-12 and around 2.1 V. I've been having lock-up problems ever since installing the second set. For far I've determined the following:

-When overclocked to 850, with voltage set to 2.0, each kit will run perfectly when alone (2 DIMMS instead of all 4), as shown by many hours of the bootable Windows Memory Diagnostic
-When run together with the same settings, my system will eventually freeze, especially when using memory-intensive applications such as games
-In the Windows Memory Diagnostic, if both pairs are installed, I will not get any errors, but the test will simply freeze before finishing one pass, always on the same test (test 4 of 6 on the standard settings)
-When the BIOS timings and voltages are set to default (no overclocking), the memory will perform normally, even with all 4 sticks installed (as opposed to locking up)

I'm terribly confused as to why these kits would run perfectly when alone at OCed speeds, but together, they fail 100% of the time. Yes, my system will boot, but both system use and memory diagnostics have demonstrated that something is wrong when I'm at 4 GB/850 instead of 2 GB/850 or 4 GB/800 of system memory.

Any help would be appreciated, and important system details follow.

EVGA Nforce 650i Ultra Mainboard
Intel Core 2 Duo 6420 @ 3.0 GHz (8x multiplier, 1500 FSB)
Crucial Ballistix DDR2 800 2 x 1 GB dual channel kit (4-4-4-12, 2.2 V rated)
G.skill DDR2 800 2 x 1 GB dual channel kit (4-4-4-12, 2.0-2.1 V rated)
EVGA Gefore 8800 GTS 320 MB

Thanks!

4 DIMM overclocking is much more difficult than 2 DIMM.

I hope you have a good power supply, helps for this sort of overclocking. I ran into similar problems (I also have a E6420 and 4x1GB Ballistix) and found I had to up the voltage for both VTT and NB once I went from 2 to 4 DIMMs.

I would set the DRAM voltage to 2.2V to assure they get plenty, those Micron chips can go above 2.3V (think those Gskills are Micron-didn't check ramlist) no problem. On my P965 board I had to bump the NB from 1.25 to 1.45V, and the VTT up to 1.35V. Do them a step at a time from stock to find your stability point. Orthos is the best program for stability testing, if it can stand 12+ hrs of Orthos, I doubt anything will crash it.

VTT and NB?

The only voltages I can set are:

CPU core
CPU FSB
Memory
"Nvidia SPP"

Is the SPP the north bridge?

Well the tip on increasing the SPP voltage seems to do the trick. Apparently the 650/680 chipsets generally won't run 4 GB without the SPP at 1.4 V. Even tech support people over at OCZ recommend doing so.